Johnson, Evinrude, OMC, outboard motor, outboard motor repair, 9.9, 15 hp, date/year of manufacture, water pump, carburetor, long shaft, 15 hp conversion, sailmaster

Maintaining Johnson/Evinrude 9.9
15 hp 2 cycle outboards

1974 – 1992   (Information & normal repairs)   

1974 Short Shaft 1987 Long Shaft

One thing that I want to convey here is that I write only on motors that I have actually worked on, and over a period of MANY years, consisting of many THOUSANDS of hours, and then more in maintaining/updating these pages.  These articles are motor specific, HOWEVER if you are having a diagnosis issue, my Trouble Shooting article covers MANY more aspects that may help you diagnose your issue.  CLICK HERE for access to that article.
   Introduction :  These articles are put together from actual experiences over a 50 years, mainly because my memory is not anywhere as good as my wife's.  I am not a marine mechanic, but have been an outboarder and have done my own repairs all my life and have been a machinist forever.  I document here what I have encountered, so that I can go back and read my own writings if something pops up years later.

   For these outboard motor repair articles, I DO NOT try to give the reader a documented list of parts needed for a conversion, (as this could be different because of running changes made even within the same models), but do try to document what differences I have encountered and tricks I have learned, many of which are not listed in service manuals.

   I have also tried to break out specific areas of repairs into separate articles, however some may overlap.  For these, you may see a slight duplication, but with a more complete write-up on the article more specific to that repair.

   My articles are probably the most complete information you will find for the 9.9/15hp Johnson/Evinrude motors, however they can be extensive and dull reading for the person who has no clue as for outboard repairs.  I often get inquiries, which I try to answer, however most of the time, the information requested is there, if that person would have taken the time to read farther.

   This article covers many aspects of these series of OMC motors, but over the years it has grown to the point that I have had to split it into separate specific sections.  In doing so, you will see a CLICK HERE or  Yellow Highlighted underlined link that takes you to the article that is specific to what is being discussed.  Some links will be to online information will have the link in an underlined blue color.

   History of This Series :   Johnson was one division of OMC, (Outboard Motor Corp) while Evinrude was the other half.  This came about by the merger of both companies in early 1956.  Both brands of motors then came from the same plant after that.  So what you see here that says Johnson, will also USUALLY pertain to the same equivalent Evinrude motor.  Evinrude was usually painted a blue color of some sorts, while Johnson was initially green until 1977 and then again white.  Late 1990's they both were either black, white or a blue gray depending on the year.   OMC filed for chapter 11 bankruptcy and  in October 2001, Bombardier Corp. a Canadian manufacturer that builds snow mobiles, airplanes plus a multitude of other vehicles bought the outboard motor division.


   Bombardier phased out the Johnson line, invested heavily into a fuel injected 2 stroke motor, but only under the Evinrude name.  Then in July of 2020 when the Covid10 virus hit recreational activities hard, they decided to drop their outboard motor line.

   The earlier Johnson 10 hp QD series motors were made from 1949 thru 1963.   In 1958 these motors had a face change, utilizing a newer style motor cowling and a change in color.   The 9.5 hp Johnson and Evinrude motors were made from 1964 to 1973.  These 9.5 hp motor's early model number was MQ up until the model code was changed incorporating the date in 1969.   If you need year of  a manufacture for these motors, or any before OMC went to the year code in 1977, as described below, go to 

   Please excuse me for focusing on the Johnson brand, while after 1956 under the OMC manufacturer, both motors were made in the same plant, but painted a different color and sold to different dealers.  It is just simpler in these articles to focus on one manufacturer, and in certain areas, there appear to be more old Johnsons floating around than Evinrudes.

10 hp QD series 1949 to 1958

10 hp QD series 1959 to 1963

9.5 hp MQ Johnson 1964 to 1973

hp motors covered in this article, came into being in 1974.   As for actual physical differences between the 9.9 and the 15 hp that you could use for identification other than the decal markings, year by year these 2 hp sizes and brands are the same.  If the top cowling was removed or lost, the name/model plate was missing, then to tell the difference you may have to remove the carburetor, measure the inside throat to tell if it was a 9.9 or 15 hp as for the layman being able to identify a carburetor by stenciled on numbers is about impossible. 

    The Johnson line of 9.9 motors remained the same as far as calling it a 9.9 hp.   However Evinrude had a 10 hp designation on some of the later ones from about 1990 or so.   Same motor, but they apparently just changed the stated rating for advertising purposes.  The reason this motor was originally called a 9.9 hp, was at the time of this motor's inception, the US Coast Guard's terminology for registering a boat was that if it used a motor of less than a 10 hp and the boat was less than 16' long, it did not need to be licensed IF not used on Federal waters.  All 15 hp motors for both Johnson and Evinrude were called 15hp.

   The piston diameter for both the 9.9 and 15 hp motors from 1974 to 1992 are 2.188" with a stroke length of 1.760"  which equals 13.2 cubic inches, or 216 cubic centimeters.  Full power operating RPM for the 9.9 is 4500 - 5500.  The 15 hp motor is rated at 5500 -7000 RPM according to a 1978 Johnson sales brochure.  This higher RPM is achievable by using a different carburetor which uses a larger throat, which gains a higher fuel/air mix.
   These were the second series of motors in this size range which were made with the compact lowered powerhead (the 9.5 being the first) and more easily fished over the top of which set the trend for other manufacturers to follow.  This series of motors appears to have also been among the first of the thru the prop exhaust motors.  The outward appearance of this motor, other than paint and decals, remained virtually unchanged for 18 years, even up until it ended in late 2006, it evolved into the backbone for the current 2 stroke model.  Both the current 2 stroke and 4 stroke motors use the same water pump and virtually the same lower unit (slight external changes) as the 1974 version.  OMC apparently learned something on the 9.5, as the 9.9 is basically a detuned 15 hp, initially using only a different carburetor and decals on the cowling.  


   Observance :   It has been noted that since I have started these articles and have gotten many e-mails from all over the world that other country's motors may not be exact to what we see in the United States.  To me this means that if there could have been an overage of production parts for a certain year, that the new model would be sold in the US, but the other countries could have been used as a cleanup of parts and you could see some differences as to electronics, decals, colors etc.  There could even be specials made to slightly different European specifications.  Or some specific things would be made available like the AC lighting for some European motors.  So it is possible that at times the year models would be a year or so behind the ones sold in the US.

    When a new design is brought out, it seems that there will surely be modifications needed to correct unforeseen defects or problems that had popped up.    However if these modifications were crucial, a running change could have taken place, with the last letter in the model number relating to a revision change.  It seems when OMC came out with a new design or a major "improvement" to the design that they are hesitant in making a quick change for 3 years.  This is evident in that the first models in 1974 used the magneto ignition system with points and coils until 1976 when the electronics went to a Capacitor Discharge system.  The carburetors seem to be changed in either 3 or 6 year increments.   And not all size of motors used the same changes for the same years.

   It is suspected that possibly they want the design to run for long enough to really perfect a better part?   Or more than likely that they needed to keep those parts in production so that inventory restocking would not become a nightmare by being changed every year or so.   If they worked on a 3 year cycle it would not be as bad and a better parts interchange, plus this would give them time to work out any "bugs" and get new parts perfected. 


 1974 (earliest of this series) Shortshaft

1992 Longshaft (last of this particular series)  Note the different tiller handle, twist grip & red kill button



   Year of Manufacture :  This is important in any repair, for obvious reasons.  Prior to 1979, Johnson used the last 2 digits of the year in the model number.  As a model 10R78M, would indicate a 10 hp, Rope starter, 1978 year of manufacture and M model revision.  Since this does not have in the code L between the hp designation and the year, that would mean it is a short shaft version.   Evinrude in those years used a different 5 digit model code, as 10424SThe numbers  you are looking for will be the first 3 numbers which equates to the hp and then the middle to the year, as 10 hp, the (4) equates to 1974 while the (S )would indicate a short shaft.  But not all of Evinrude's pre 1979 models were not that simple.   Here is a LINK to help you determine any of the Johnsons.  And here is the LINK for the Evinrudes.  For those of you readers outside of the United States, here is another LINK that may better help with your motors.


   After 1979, OMC's manufacturing year code, which included Evinrude, is the word "INTRODUCES", with each letter equating to a number ie:  I=1, N=2, T=3, R=4, etc. as illustrated below.  As before, the last 3 letters are what you are looking for.  Then in this case, disregard the last one, as it pertains only to a model revision that only on specific cases may be reverent to a marine repairman if problems arise.  A code of  J10ELENA would equate to Johnson, 10hp, Electric start, Long 20" shaft, EN = 1992, and the A could be a model revision, so this motor would probably be made in the very first part of the year. 


   Now if the model number was J10SELCTC this would have been  Johnson, 10hp, SailMaster, Electric start, Extra Long shaft, 1983, and the C could be a model revision.   In this case with the SE in the model number, indicating a SailMaster.  All the SailMasters would have all had the Extra long 25" shafts and electric start.   At one time Evinrude's equivalent of the Johnson SailMaster would have had the name Yachtwin.  These were designed to be used as an auxiliary power for sailboats, hence the extra long shaft.   


   You may also encounter a code of something similar to J10RELCTC, here the R would mean a factory Remote setup.


   Using this code system, if the motor was sold as an Evinrude, then the first letter would have been an E instead of the J for a Johnson  Then you can encounter motors made in or for other countries, which will have some added letters as mentioned later in this article.  However some of these foreign motors could have model numbers harder to decipher.


    For more detailed information on worldwide production, CLICK HERE
























   There should be the model and serial number on riveted on 1” X 2” aluminum ID plate on the LH side of the transom mount bracket (looking forward).  Sometimes this ID plate can get removed.   If this happens, you can still usually tell which motor it is by looking at the 25 cent size soft plug in the upper rear RH side of the block.  The model and year are stamped in this soft plug also but probably only up to 1979 in a code we can relate to.  It is interesting to also note that the 15 hp uses the same block as the 9.9, (just a different carburetor) so you could see a 15 hp with the same 10R78M numbers here as the 9.9 has.   I have heard from more than one person that their plug had numbers that do not match any of the codes, so this kind of backs up the idea of a change after the 1979 date, as the factory changed things as time went on.  However another reader said his plug codes were good at 1988.  ??


   If you need to check on parts and do not have a actual parts list booklet, one of your best sources is   They have a complete listing of about all of the Johnson and Evinrude motors showing exploded parts views with part numbers where you may be able order parts online from them.  Or this one

   In 1981 the marine industry went from powerhead rating to a prop rating.  The word was that the imported Japanese engines of the same rating were outperforming the US engines, as they were rated differently.  To get the equivalent prop HP on a US motor prior to 1981, take the HP rating then subtract about 10%, this will get you close to the newer rating.


   One thing that I have found is that the pre 1977 motors which have the points and condensers seem to be a little cold blooded, in that they have to warm up before they will stay idling as for trolling.  Also do not knock these pre electronic ignition models.  Many boaters are not interested in these motors, preferring the electronic ignition, and I have to say if I had a choice I would also, but the older point system has performed quite well until modern technology came along.  They just require a little more prolonged maintenance.  And as they get older and more obsolete, in isolated parts of this planet, they are the ones that can usually be kept running by persons who have any degree of understanding magnetos and are mechanically inclined.

   Controls from 1974 to 1985 :   This series of motors have all the same basic controls.  That is on the front panel, the choke pull knob is on the left side looking back to the motor.  To the immediate right is a large (approx 1" dia) knob that rotates, but will only go about 1 turn.  This is the carburetor idle adjustment knob.  In the center is the manual starter pull handle.  On the far right will be the kill button.  If it is electric start, on the left side looking from the front, hidden in the mounting base is the start button.

   In Europe, at least from 1979 to 1983 manual start 9.9hp motors uses a lanyard man overboard kill cord BUT the clip is attached to a different secondary kill button which is located in the normal location as the electric starter button located in the US.  This lanyard clip, when pulled out kills the motor.  The regular kill button is still in place and functional however.  This same motor has the AC lighting plug on the port side like mentioned farther down in this article.

Front panel, showing choke, carburetor idle adjustment, rope pull handle, electric starter button & kill button Here is a 1985, notice the smaller manual starter knob & the red ring around the stop button 


   On the left side looking forward, is another large black knob located immediately behind the fuel line connector.  It is the slow speed control screw.  This knob DOES NOT have anything to do with the carburetor idle jet setting.  In effect this is basically for setting a slow trolling speed, where you may want to return to after making a run and without playing with the twist knob setting.  In setting this one, you MAY have to adjust it and the carburetor idle knob simultaneously to fine tune each.

   Sometimes (especially after many years of throttle gear teeth meshing and with a lot of wear), it may be nearly impossible to get this slow speed control to effectively function.  Therefore over time, it's use may get abandoned.

   In the RH photo below, you can see the flat lower shaft is held from rotating by the flat angled Ell on top, which is also shares the fuel line couple retaining screw.  In this photo, the throttle is in the SLOWEST position as shown by the white wedged part (the far RH) of the throttle linkage, which the flat control shaft can be adjusted to stop against.

Slow speed control & fuel line connector Slow speed control looking down on it

   This slow speed control knob is prone to abuse where the snap ring retainer groove can get broken which allows this knob and stop screw to become inoperable.  It is unique in that the threads are LH, this is apparently so that by turning it to the right increases the speed setting.  

   One cure for it without purchasing a new knob is to do some measuring and eliminate the wafer washer under the snap ring.  Put the knob in a metal lathe, then carefully move the shank back into the knob area, deducting the .050 of the washer, recut the snap ring grove.  Shown below are the important dimensions.  The purple lines are what was removed.

The damaged idle speed knob modified & salvaged

   On the right side again looking forward is the shift lever which is evident in the photo below.    Also below the shift lever but just above the mounting brackets is a lever that moves forward and back.   It locks the motor down so that if you put it in reverse and try to attain a high speed, that the lower unit will not jump up and out of the water.  This lever, when moved forward, allows you to raise the motor, lock it in the up/out of the water position.  Also in the same 2 photos below, you will see the black shifting lever described just previously.  And the upper latch cowling lever is located at the rear and under the edge of the cowling, rotating it down allows you to remove the upper cowling to access the engine compartment.

RH side showing shifting lever and tilt lock lever on a 1985 Here you can see both the outer lever hanging down & just the end of the inner part of the latch on this 1992

   For a detailed description of the throttle linkage and it's associated problems


   Controls from 1986 to 1992 :   Apparently in LATE 1985 a change was made to the tiller handle/throttle twist grip, which I call this version the type 2.   This cable system replaced the gear throttle linkage improving the situation dramatically.   At this time they also moved the kill button to the end of the tiller handle.  It has the word STOP on the end.   In 1987 they added the lanyard type kill button on the end of the twist throttle handle  These created a problem if you used the extended universal joint tiller handle, as you could not reach the kill button with this extended handle installed.  


   In 1991 the handle was changed slightly and the kill button was moved to about mid handle left side.    These buttons for both type 3 and 4 also incorporated a man overboard kill switch.  The motor comes with a red coiled plastic cord that is attached to a split plastic section that is inserted UNDER this kill button.  The other end of the cord is to be attached to the operators belt or wrist, so that in the event he falls overboard, this insert is pulled out and the spring loaded red button goes deeper in the housing just like turning a switch off (product liability).   In use, this electrical circuit is just reversed from what most of us think, as with the clip inserted, turning the switch on, you are actually breaking any electrical connection.  To kill it by either the clip missing or by you pushing the button, you connect the wires, grounding out the wiring, killing the motor.

   In use, this red button HAS to have either the lanyard insert clip under the button, (turning the ignition ON) or another smaller one called a "restart clip" shown below on the 2 photos on the right, that is inserted instead of the lanyard if the owner decides he does not want the lanyard safety device.

   On type 2, 3 and 4 the ribbed plastic end plug is made to rotate.  This is to fine tune adjust the idle for trolling, as these motors do not have the LH side trolling adjustment knob.  In the photo for type 3 you will see a black "U" shaped clip  attached to the top base of the handle.  This is spare that was original equipment.  Another like it had the red coiled plastic looped cable with a clip to attach to the operators arm.  In the RH photo you will notice a lighter red colored "U" clip under the end button.  The angle of the photo for type 3 does not show this clip well.  This is the restart clip as described below.

   The type 1 twist grip may have different grips depending on the vintage of the motor, the early ones being a white plastic.  The electric starter button remains in the same position on the LH side in the mount clamping area for all these motors as the previous versions up thru 1992.

Type 1,  gear type, kill button is on the front RH corner of the motor cowling Type 2,  cable type, black kill button on end, no man overboard switch Type 3,  cable type, red kill button on end, but with man overboard switch there also Type 4,  cable type, red kill button & with man overboard switch farther back on side

Years 1974 to Mid 1986

Late 1986

Years 1987 to 1990

Years 1991 to 2007

   As shown above, on motors made from 1987 and later which have the kill button on the tiller handle, if either of the clips shown below are missing the motor will not start, kind of like turning a key OFF.   There are 2 variations of this, the one shown is from a 1987 to about 1990, which as it on the end of the twist grip throttle handle.  After that, the clip was moved to a separate switch about 1/2 way up the handle, while retaining the smaller twist knob as a slow speed limit control.


OMC's illustration for restart clip for 1987.    This button will be moved rearward for newer motors

   The above the following information are a reprint of OMC installation instructions for "OMC Clip Assembly P/N 431808" dated 3/87.  This part is also known in the parts manual as (Restart Clip).  "This clip assembly is for use, under limited conditions, on models with an Emergency Ignition Cut-Off
 Switch located in the steering handle.  Make sure that you are aware of the purpose and benefits of the Emergency Ignition Cut-Off  Switch  as described in your Owner's /Operator's Manual before disabling.


   Outboard motors used in certain applications, e.g. auxiliary power on cruising sailboats or as a trolling motor on larger fishing rigs, may NOT be able to take advantage of the benefits  provided by the Emergency Ignition Cut-Off  Switch feature.  For applications such as these, the cut-off switch can be disabled by replacing the clip and lanyard assembly (A) with the OMC Clip Assembly (B), as shown.  The clip (B) will disable the cut-off switch while retaining the "PUSH" Stop Button feature.  If the motor application changes, reactivate the cut-off switch feature by replacing the clip (B) with the original clip and lanyard assembly (A)".


   The Type 2 thru Type 4 made from late 1986 to 1992 use a cable system for the throttle advance (as shown below) instead of the old troublesome gears system.  The rear end of the cable is threaded for adjustment to the Nylon coupler that snaps over the steel ball on the pivoting connector that is in turn pinned to the timing plate arm.  This threaded end can give some final adjustment in addition to the twist grip plastic end cap adjustment for slow speed idling.  However it has been found that when threading this Nylon coupler on, rotate it slightly upward on the outside, otherwise it can become uncoupled if situation s are slightly wrong when you move the throttle to slow position.   For more detailed info on how to adjust this or adjust the slow speed stop,  CLICK HERE.

Here is shown the throttle cable routing thru the lower cowling Here the arrow is pointing to the ball socket on the end of the cable connected to the timing plate arm unit


   The real problem that I ran into here was that with the above adjustments set so that the throttle markings, SLOW, START and FAST were aligned where you would think they should be, with the throttle roller being at a position where the timing plate mark would just start to move the carburetor roller, the timing plate was out of synchronization for maximum open throttle which would not perform a optimum high speed.  My solution was to adjust it so I had my maximum high speed, which still gave me my idle speed, BUT with the throttle indicators being off (way fast like actual slow speed running was just faster than the START position).  This positioning also screwed up the end of the throttle handle slow speed adjustment to where it would not function.


    Aftermarket Emergency Lanyard Kill Switch :   For those of you who want to add on the emergency kill switch especially if allowing younger boat operators, there is a aftermarket one that is a reasonable price and easy to install on most outboard motors.  It is Sea Dog 420488-1 Kill Switch with Lanyard that sells for less than $10 and available on  This is designed so that it can be installed on the older magneto ignition or the newer electronic ignition.


Sea Dog 420488-1 Kill Switch with Lanyard

    The Motor Will Not Start :  On these post 86 motors, I also have had the ball socket (red arrow above) become disconnected occasionally when you moved the throttle to SLOW (even after installing a new Nylon coupler).  In this situation, the motor will not start, (it may try, BUT not for long) as your timing plate is positioned at the stop position and no amount of fuel you try to impose will even get it to do more than try to start, but then fail again. 


   This can tax a trouble shooting situation because you are usually looking at the other side of the motor and may only find this after doing a spark test and then maybe a carburetor rebuild which neither will improve the situation.   And you can not keep it running long enough to to a pull the spark plug wires to see if it is running on only one cylinder.  After finding this disconnect, my Red-Neck solution was to Zip Tie the coupler to the timing plate arm.

Here you see the Zip Tie solution for the ball coupler coming loose

   How to Start Your Motor :
  I get inquiries at times and some owners do not know or understand the basics of starting an outboard motor, so.  The first series in 1974 had a nameplate riveted onto the bracket just below the control panel, stating:   "Starting Instructions:  Read owners manual.  Connect fuel line to motor.  Squeeze primer bulb unit pressure is felt.  Shift to neutral.  Twist grip to start position.  Pull choke knob out.  Pull starter handle.  Push in choke knob as engine warms up.  Stop button on control panel.  Spark plug UL4J.  OMC  sea-lube gearcase lubricant."

   Now one thing to remember is that usually the throttle labels on the tiller are very vague and are more a reference point than actual throttle position.   This said, a motor WILL START, IF the actual position is a little faster than the indicated start position on the twist grip, BUT may not even sputter if set to slow.  However when shifting always try to be as low as possible to help alleviate a lot of gear grinding.

   Engine Has Sat For a While :  
If the engine hasn't been used in quite awhile, very possibly with gas sitting in the carburetor, it may possibly require cleaning, which seems to be the standard recommendation for motors in this situation.  However, it may be worthwhile to try to run it at least once before jumping into a carburetor repair.  I have found that sometimes the gasket between the main carburetor body & the bottom fuel bowl can deteriorate.  When this happens, for what ever reason, chunks of the neoprene gasket get into the fuel bowl and can partially plug the high speed jet, cause erratic engine performance.  It has also been found that the newer non-leaded gasoline is less likely to cause internal gumming problems in the carburetor if allowed to set for extended periods of time.

   The only way to fix a clogged/fouled carburetor is to remove, dismantle, clean, and rebuild it.  Do not try to spray a carburetor cleaner in the breather or run it thru the fuel and expect it to do a cleaning job.  If you try this while the engine is running it very well could ruin the engine, as there would be little lubricant inside the engine for the bearings.  If you take the carburetor apart, make notes and drawings as to what goes where, or use a digital camera.  Regardless of how simple it seems, you will find something where you can't remember how it should be when it comes time to put the parts back together.


   The One Weak Thing on This Motor :  The first series (that I designate as type 1) up to and including 1985 uses twist grip gear teeth to activate the throttle on this series.  As described above these gear teeth  mesh on both the twist shaft and the mating  linkage shaft gear function so that the handle can be raised or lowered and yet be twisted.  BUT this feature was not really intended to be used this way as mentioned earlier under "Remote Controls".   DO NOT RAISE the handle very high AND twist the throttle unless you are prepared to disengage the gears, which changes the twist grip timing relationship.  This is explained in a subsequent section.  Pictures show this relationship in the Trouble Shooting section.   The main problem occurs if these motors are being used as an auxiliary motor on a sailboat, where the tiller handle has to be tilted upward and WILL get out of engagement easily.  I have looked at this situation a lot and have not found an easy method of correcting the problem.


   Later, in about 1986 a throttle cable push/pull system was used to eliminate this problem.


   Difference Between the 9.9 & 15hp :  When they designed this series of motors, they designed it as a 15 hp and then detuned it with a different carburetor for the 9.9.   When the 9.9hp and 15hp came into being in 1974 and up thru 1978, the only real difference between the two motors was the carburetor.  In 1979 they added a shim, part #325038, (one for each cylinder), behind the leaf valves and under the stop plate for the 15 hp, apparently to allow leaf valves to open up more at higher RPM, giving the motor more fuel in the air mixture. 

   The owners manual indicates that the operating range  RPM for the 9.9 to be 4500 - 5500 where for the 15hp it is 5500 - 6500 RPM.   So where the 15hp gets it's power over the 9.9 is that the carburetor allows more fuel/air to increase the RPM by 1000 which equal the extra power.

   It is my experience however that the 15 hp does not idle down as slow or as smooth as the 9.9 does.  It should not really make that much difference as at an idle they use a different idle jet than the main jet, except that the 15 has a larger throat size and apparently needs slightly more fuel even at an idle (trolling speed).  So if you intend to troll for trout at a SLOW speed, stay with the 9.9 carburetor, however if you want it for a backup motor and salmon troller (which usually requires a slightly higher troll speed) then consider the 15 hp.  The change to aftermarket Boyeson reed valves may help in this situation however.

   The outside visual dimensions of both carburetors appear the same, unless you look INSIDE the throat.  If you look in from the rear, you will have to position the throttle plate to a horizontal position to be able to see the throat very well.   The 9.9 hp carburetor’s INTERNAL THROAT is .625 dia., 15 hp internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor.    NOTE this is not measured from either the from or back but the internal throat that you can see on the LH photo below.   The outlet or rear throat dia. is the same on both to match the manifold port dia.   The idle jet venturi system is also slightly different between the two.  The part number for the the pre 1987  9.9 hp Carburetor Assembly  is #388936, while the 15 hp Carburetor Assembly is #338273.   This increased throat dia. and larger main jet allowed it to breather better and to pull more fuel in, increasing the RPM about 1000 RPM, hence more HP. 



    For those of you who think you are a good machinist and can bore out the throat of a 9.9 to make it a 15hp, well NOT REALLY.  The throats are made different, you may be able to take MAYBE ¼ of the needed diameter, but would you get any noticeable gain?   Then the mainjet tube from the float chamber up thru the center of the throat and into the upper section is different on the 15.  And I have NEVER been able to remove these tubes without ruining them (or getting so close to it that I gave up), so without being able to do that, you could not bore it out.   Try it if you want and let me know how it comes out, but be prepared to buy another carburetor as your backup plan.

9.9 on left & 15 hp on right as viewed from front, & with choke open, notice the internal throat size differences 9.9 on right & 15 hp on left as viewed from rear, but with throttle plate closed & you can not tell any difference


Looking in from the front behind the choke, the light blue is the throat size for the 9.9 hp, while the dark blue represents the 15 hp carburetor, represented by this full size CAD drawing of a pre 87 carb.

   A problem in buying a used carburetor is that the part number may be hard to read as it is only stenciled on.   And many people, even marine mechanics, do not see enough of these to be really sure which is which unless they see them side by side.   The reason for this is that when these motors were sold new, say in 1978, the 9.9 went for $647.75, while the 15hp was about another $150.   Needless to say way more 9.9's were sold than the 15's by possibly 80%, therefore considerably less used 15's parts are available.


   FYI, here are the specs on the carburetor mainjet/orifice = 9.9hp Part Number 321742 - ID Number 40 - Orifice Size 0.040 inches - 5/16-24 thread size, 15hp Part Number 322293 - ID Number 54 - Orifice Size 0.054 inches - 5/16-24 thread size.  Either one will screw into the other body, but will not function unless the throat size is correct for the HP.

   If you do decide to up you HP by changing to a 15hp carburetor, try to stay in as close a a year range as possible because there were changes in the carb arm cam roller diameter.  As the replacement carburetor you get may have a different size roller, you will have to "link and sync" the carb roller to the timing plate, otherwise you may not achieve the high end performance that you anticipated.  Or swap the carburetor roller arm assembly off your 9.9 to the 15hp carburetor.

   In 1981 there was a change made in the inner exhaust tubes for the 15 hp OMCs only.  It was a sort of tuned round exhaust housing, allowing the engine to again breathe better, which  in addition to the larger throated carburetor, and the leaf valve shim gained about another 10%  in overall HP to put these OMC engines on par with the competitive Japanese imports.  The 9.9 hp square inner exhaust housing with no shim for the leaf valve, stayed the same as it was.  A guess here is that they needed to use up the older parts and at the same time have a noticeable difference between the two HP motors.  The big difference in the performance between the 9.9 and 15 hp will be in the last 1/3 of the throttle range where the increased fuel/air flow of the carburetor will be readily apparent with the increase in RPM.  

   In the photos below, the early 9.9 and 15 hp exhaust tubes are on the far left, with the round tuned 15 hp on the right in both photos.  The copper tube on the tuned one is so the engine will disperse the exhaust water more down than as compared to the older 74-76 motors that needed the water exhausting higher to be able to exit the blubber hole before the overboard water indicator (pee hole) was used.

RH side view of 9.9 & 15 hp exhaust tubes Early 9.9 exhaust tubes on left & later 9.9 & 15 hp "tuned" exhaust housings on right, top view


   Later motors, at about 1990 the 9.9hp uses the 15hp exhaust housing.


   For an explanation of possible water tube grommets deteriorating where they attach to these exhaust tubes, possibly causing water blockage CLICK HERE for a link to my water circulation problems article. 

   You will note that there were 3 different ONGOING 15 hp modifications.   (1) The 15 hp from 1974 to early 1987, with just the different metal topped carburetor.  (2) The added leaf valve shim in 1979.  (3) And the added tuned exhaust system in  1981, which also included the 2 previous modifications.  The 1979 parts manual lists a shim, part #325038 to go between the leaf valve and the stop for the 15 hp only.  This shim sells for $2.94 each.  It was carried on up into 1992 for the 15 hp, which apparently allows the 15 hp to breathe a little better.   These leaf valves are the same for all motors they are used on.

Below is shown a full size 15 hp shim that is .0125" thick


   To  reiterate, when doing a conversion from 9.9 to 15 hp, yes you could just change the carburetor, and depending on the year of motor you may have, it may equal what the factory did for that year.  But if you want to do the whole thing using information they developed over the life of these motors, you may also want to add the reed valve shims AND the later round tuned exhaust that was developed for the 15hp.  Obviously if you are converting a late motor, say a 1990, the reed shims and round exhaust should already be on your motor.

   If you are decided to install the aftermarket Boyeson reeds in these motors, you DO NOT use the 15 hp or later 9.9 spacers because the Boyeson reeds are a dual reed system with a primary and secondary reed that stack on top of each other.  The primary reed has a hole in the center.   And they DO NOT come with new reed stops for these motors, so you need to use the existing factory ones.   

 Boyeson reeds installed in the reed plate

   Now before you run out and try to find 15 hp parts to convert your 9.9, here are some drawbacks.  I have 2 friends that have these 15 hp motors and both essentially for trolling.  The problem that both encounter is that they foul spark plugs more regularly than if the motor was a 9.9.  I suspect that could be from 2 situations.  (1) Here both say they are using 50-1 oil mix.  This is combination recommended, but you will notice in this article that I have experimented with the ratio and oil type and have found what I consider a better combination.  My combination would extend their run time before changing plugs, how much, I am not sure.  (2) Since the carburetor has a larger throat, in my mind, the idle jet would have to allow more fuel into the engine to give the proper fuel to air mix otherwise the engine would be running lean.  With this slightly enriched idle mixture, this could explain the more frequent plug fouling.


   Therefore possibly changing to the aftermarket Boyeson reeds would be a benefit, however I have not done that on my motors.


   Hard Starting :  These motors can be hard to start unless the choke is staying fully closed while starting.  Verify that the stainless steel tang that holds the plastic choke lever in place is doing its job, after a while they need to be re-bent back in place to apply enough tension on that choke rod notch to hold the choke in position.  Also verify that you have replaced the choke knob right.   If your choke knob shaft was installed upside down, the choke butterfly will not completely close, thereby not being able to choke the engine when doing  a cold start. The wider slot on choke knob shaft (the connection point to choke butterfly lever) goes up.

   Some motors start well with the idle knob (lean/rich) knob set in a normal idle position, others may require enrichening the idle setting for starting.

   Hard Starting When Cold :  The one most common problem with hard cold starting, if the motor is in reasonably good running condition, can probably be contributed to someone who uses the motor strictly for lake trolling and is prone to adjust the idle jet lower than it is actually designed for.  When you are adjusting this timing and idle jet for slow trolling, you will be doing so while the motor is warm and of course running.  Then when the motor cools down, things are going to be slightly different.  What happens then is that in the cold starting mode, the motor does not get enough fuel, and you have to choke it and pull more times on the starter rope to get enough fuel into the cylinders for it to fire. 

    One thing you may do in this case is to after you have adjusted it for your trolling speed is to then pull the knob straight forward and off the carburetor idle shaft, reposition the idle knob to position the pointer straight down and push it back on.  This gives you a pretty well defined "Go To" position.  Then when you go to start it, you twist this knob counterclockwise all the way up (about 90 degrees) till it bumps.  This should give you slightly more fuel to start, and then you can reset it back to your known trolling position later.

    Another problem could be leaky reed valves at the reed valve base.  This could allow some of the intake fuel to be pushed back out on a firing cycle.

   Another possibility is that the engine may have worn/leaking crankshaft seals.  This condition will not allow sufficient vacuum on the intake stroke to pull fuel from and through the carburetor.  This is usually that the top seal the one that is bad.  Some mechanics will tell you that it is the bottom crankshaft seal, I say not really, as the lower one will not have as much side pressure on it as the top seal being near the rotating flywheel as the top bearing may also be worn.  These leaky top seals will usually make a mess under the flywheel, namely fuel/oil mix spilling down around the block.  My simple method of doing a check out of the top seal leaking is to pour a small amount of STP around the shaft/seal juncture.  It sure can't hurt anything and just may give you an indication as to if that is the problem.

   Recently while talking to a couple of duck hunters who experienced a OMC 15 hp  THAT REFUSED TO START in freezing weather.  They mentioned that one COLD morning, they were not able to go out to the hunting grounds because of the non-starting motor.  This was devastating as they were on a trip a considerable distance from home.  A trip of "let your fingers do the walking" thru the phone book took them to a old outboard mechanic that was operating out of his home shop.  When they explained the problem, he said OK I know what it is.  When they looked him up, he retrieved a propane torch and heated the spark plugs.  After reinstalling them, BINGO, the motor started.  His explanation was that on some of these older magneto ignition motors, the compression may be lower during the cold weather and that there just is not enough compression to fire the fuel at the plugs with them being that cold.  WOW, sometimes the old guys do rule.


   Carburetor, Air Breather :  No, these boxes mounted on the carburetor's intake are called a silencer.  There is nothing inside these silencers as that's all they do is silence (there is no dust on the water).   It's like a muffler for the carburetor.  Don't even think about adding sponge or anything else to the silencer as parts may get sucked internally into the motor, NOT GOOD.


Here you see a round hole in the rear of the silencer (air breather box).  I can find no explanation for this other than it may be a mini supercharger of sorts, relying on the rotating starter gear teeth as a fan when the motor is running 


   Carburetor, Early :  Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet.   All you accomplish is that the engine is being fed too much fuel and not enough air to properly mix with it, the engine will then blubber (floods out) at the top end.   It can run OK on the lower end or idle OK because it is using the idle jet then, but it will have less top end power than with the original 9.9 main-jet.   Believe me, as I have been there/done that.  Matter of fact that is what years ago got me into writing the original of this article so I could document and remember myself, as I could not readily find any factual information on the subject, only speculation.  To reiterate the above, a 15 hp main jet installed in a standard 9.9 carburetor WILL NOT work.

   Early production motors had all metal carburetors.  Then from about 1980 to late 1987 the same basic carburetor as previously was used, but had a black plastic bottom fuel bowl.  This plastic seemed to be an improvement in that if water had entered in the system that then settled in the bowel, and sat for a while creating RUST inside the carburetor, even enough rust on the outside if used near saltwater to corrode holes in it
For a more complete information on the carburetors of these motors CLICK HERE.

   Carburetor, Late :  Late 1987 and newer production carburetors up thru 1992 were completely different having the bottom fuel bowl and top made of heavy black plastic type material.  The choke lever is the same in these newer carburetors even though the motors used the cable twist grip which utilized a totally different throttle system.


   They were apparently designed for easier manufacture as the plastic top has the idle jet in it along with the juncture between this top and the main body serves as a passage (depending on the the gasket) for the fuel from one chamber to the other. 

   The choke butterfly is the same as the previous models, but the throttle plate is smaller.  The internal throat of the 9.9 is about .500 dia. while the outlet hole into the manifold is .750 dia.  


    If you see a carburetor advertised on e-Bay that does not have the above RH side choke lever, but only a shaft protruding with a roll pin installed, it will be for a 1993 or later motor, as the choke lever is different for these.


   There then was another twist that you may encounter, that is a remote controlled motor that has a electric choke.  On these there is a small solenoid with a wire running up to the choke lever on the carburetor for activation when you push the remote choke button.

   Removal of the Carburetor :  You need to remove the plastic carburetor intake cover (OMC calls it a silencer).   The early ones (pre 76) I have seen are white instead of the common black color that is on the later versions.  It just could have been that some  motors that had white ones that were made of a material that did not survive vibrations as I have seen may missing.  Remove the choke lever and the idle knob.  When removing the idle knob, make note as to the location where the indicator tit is pointing, as you should replace it in the same location when reassembling.


   Removal of Fuel Line :   You also need to remove the recoil starter unit.  The manual starter unit can be unbolted with the one 9/16” headed bolt head in the center of the recoil unit.  The rope or knob does not need to be removed, as the unit can be pulled out slightly and laid in a location out of the way.  DO NOT remove the center bolt, as it needs to be left in to keep the unit from exploding.


    If you need to remove the knob for a complete unit removable, be very careful as that spring is dangerous!  Pull the rope out a few feet and tie a slip knot in it so that it can't rewind back.  Carefully remove the starter handle, then grab the rope inside at the starter pulley, then let the rope withdraw somewhat but not completely (leave a foot or so to work with).  Now, secure the rope inside at the starter pulley to the upright pulley gear guide in a manner that will prevent the spring from rewinding the cord etc.  Hold the manual starter assembly together when loosening the top large retaining bolt (do not remove that bolt completely from the starter housing assembly).  Carefully remove the starter assembly while holding it together.  Screw a 3/8" nut on the bottom of that large retaining bolt to hold the starter together while you're working on other components.

   There should be a small coil spring (#1 in the photo below) from the carburetor to the ratchet of the starter and up to about 1979 that needs to be unhooked.  This spring's tension is released by a lug that the timing plate cam positions when the twist grip and shifting lever are in neutral.  If the throttle is set faster and the motor is in gear, this black plastic dog drops down into the starter spool teeth, not allowing the manual starter to be pulled.   This dog's function will still operate by gravity if the spring #320490 is missing, but not as positively.  This spring is obsolete and only available from aftermarket suppliers at a cost of $9.95.

   There is also a Z link rod (#2 below) This ties the roller bar unit to the throttle valve of the carburetor.  Do not loose it either, or a makeshift one will need to be fashioned as this #2 is below from a piece of welding rod.  The important thing here is to get the bends just right so that the timing is consistent when the timing plate moves this roller forward to give movement in the throttle valve increasing fuel flow.


Spring & linkage rod location on early motors


   Now you can get to the RH (looking forward) carburetor nut more easily.  The LH nut is harder to get to and you may only get the nut to move one flat at a time with the wrench.  You may also decide that by removing the flywheel it is easier to get to this LH nut.

    Shown below is a modified 3/8" combination wrench that has been opened up to 7/16"  with relief cuts made on the sides specifically for use on these carburetors.   Removal can be done with regular 7/16" wrenches, but is cumbersome.  The reason for using the 3/8" wrench was that it being smaller, required less relief cuts to be made.

Modified carburetor wrench


   Clean & Rebuild the Carburetor :   You CAN NOT clean the insides of a carburetor by simply running the motor and spraying a cleaner into the breather.


   Disassemble the unit.  Once the carburetor is off, unscrew the bottom screws that hold the sheet-metal or later plastic float bowl on.  When removing this bowl or pan, be careful to try to save the gasket if you are not going to get a repair kit.  Remove the cross pin that holds the float, remove the float and the needle valve.  You will need to also remove the idle shaft screw, turn it in until it lightly stops, count and remember these revolutions.  In this way you can replace it with the same number of revolutions when you reassemble it.  Normal screw setting of this (lean/rich) idle screw will be starting at 1 1/2 turns out from lightly bottomed out.


   If the insides are dirty, you can get a can of carburetor cleaner, spray on or soak until things soften up.  Usually you can use an air hose to blow thru the passages to be sure they are clear.


   The older floats are made of varnished cork and can get deteriorated over time and may not function properly, if so replace it with the newer black plastic type.

    In rebuilding your carburetor using a kit; make sure you remove welch plug (a small 3/8" dia. aluminum plug) on top of carburetor and get the idle circuit holes absolutely clean and blown out with compressed air.  This plug can be removed using a ice pick.  You will ruin it, but try not to damage the carburetor body as the new plug needs to seat airtight.  I have even used the old plug, by tapping it back in place and JB Welding the ice-pick hole up.

    Pre-adjust the idle needle (the one in front) to about 1 1/2 turns out from lightly bottomed out or at the number of turns that you counted.

   Dies When You Put it in Reverse :   If the idle speed is too low, and you have it on a boat in the water or in a test tank, it will stall when you put it in reverse from an idle.  It takes very little power to run in neutral.  It takes more power to run in reverse, (because of the propeller now running backwards) so if it is idling too slow, reverse is the gear that will cause it to stall as compared to going into forward. 


   If the idle speed in a test barrel if set correctly, 675-750 RPM in forward gear, you will probably see something like 800-900 in neutral, and 600 in reverse.  But if it is set it for 600 in forward and then reverse will drag it down to around 500 which is to slow to run.


   Removal of Intake Manifold While on the Block :  It is very unlikely that you need to do this, but just in case here are some pointers.  It is just about as easy to remove the powerhead from the midsection as try to just remove the manifold.  The bottom 1/4" bolts are a tight location to access.  While the carburetor is off, you will need a 1/4" drive speed handle, preferably a 7/16" wobbly socket, (one that is a combo universal joint and socket combined).  You may be able to squeak by with a universal and a thin socket after taking out the front motor mount screw, which will allow you to gain a slight amount of room.  The hard ones are the RH and LH bottom, as the RH bolt is tucked under the support bracket.  The LH one you will have to remove the bolts from the shifter shaft, slide it to the left enough to get the socket on the bolt head.  On motors made in the first 2-3 years of this model, I have found that these bolts are indeed a 1/4" course thread, but using a head size of 3/8"

    You may be able to twist the manifold enough to get it up and out from under the throttle cam plate.  But in reassembling when you have the gasket in place, it may well be best to remove the stator and timing plate.  In doing this you now can position the manifold and gasket in the proper place without disturbing the gaskets location.  Otherwise you may be taking it off again, as there is a fuel drain hole in the bottom of the manifold that allows excess fuel to drain back into the crankcase, that if not sealed good with the gasket, will allow gasoline to dribble out, down along side the block and upper housing.  

   Check the leaf valves for tight closing, (commonly called reed valves) or replace them.  When reinstalling or replacing these reed valves be sure that you position them so that they are centered over the holes so no leakage can occur around the edges.  Clean the old gasket and use a good gasoline resistant sealant on the new gaskets, reassemble in reverse order.   Be sure if you use new gaskets that you use the right ones, as here you have 2 different ones on each side of the reed valve plate.   If you get them mixed up, the wrong one has a smaller size opening on at the reed valve area and WILL restrict the valve movement to where the motor will not be getting any fuel into the motor itself.


   Reinstall & Adjust the Carburetor :  Before you place the carburetor on the manifold stud bolts, place a small rag under where the carburetor will ultimately be.  You WILL drop at leas tone of these nuts, and there is a hole under the carburetor that encases the shifting rod.  This hole has eaten many carburetor retaining nuts (1/4" X 28NF).  It is best to use a mechanic's Pickup Tool to position the nuts in place, then using a flat bladed screwdriver to put tension on the front of the nut so you can start it on the threads with your fingers.


   After reinstalling the carburetor to the intake housing and you need to reinstalled the choke pull knob.  The choke knob shaft has a slot in the middle that the carburetor's choke lever goes into.  This shaft needs to be installed with the longest length of the slot upwards, otherwise the choke lever does not have enough room to function.  Now you need to re-install the plastic carburetor intake cover/silencer.  The idle knob is next.  Even after you replace in the same location as it was before tearing it down, and you get it running again, you may need to fine-tune adjust this idle knob for trolling.


   There is also a large knob on the starboard side of the lower cowling on all but the very early years motors.  This large knob activates an internal threaded rod that acts as a stop for the throttle plate linkage and is used for a slow speed setting, (not to be confused with the actual carburetor idle settings).  After 1987 this knob was discontinued when the cable throttle system was instituted.  The new slow speed setting was then changed to a twist knob on the end of the twist throttle.

   After the motor has been run enough to have it warmed up, try to let it idle.  If it runs to fast or slow you can adjust the large knob stop screw on the motors left hand side.  It may be best to initially do this with the motor cover off, so that you get an idea of which way to turn this screw.  Turning this knob counter-clockwise pulls the threaded rod out & makes the motor run slower.  


   When you get the motor running near your trolling speed, then turn the carburetor idle knob (lean/rich) clockwise a slight amount, let the motor run for a few minutes, then rotate it slightly more in the same direction again.  Doing this you are making the motor run leaner.  Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting and the smoothest the motor will run at slow speed. 


   It makes sense when shutting down these motors using a single carburetor for what may well be a extended period of time, to disconnect the fuel line and run the motor unit it dies, using up all the fuel in the carburetor.

   Carburetor/Spark Timing :  

If you are having problems getting the motor to run as you think it should, possibly where the timing plate is considerably advanced, you may consider doing a "Link and Sync" of the carburetor throttle arm roller to the timing plate cam. 


   CLICK HERE for a link to the carburetor/fuel pump section for more detailed information.  This article has a detailed section on how to adjust the post 87 throttle linkage timing.


   Fuel Pumps :  These fuel pumps are normally a pretty trouble free mechanical device mounted on the RH side of the block below the flywheel.  They are a diaphragm type pump which is activated by vacuum created below the pistons of the motor thru a hole into the inside the motor’s side plate that leads into the crankcase.  There is a filter screen on the outside of the fuel pump retained by one center screw that you may want to look at occasionally. 

    The primer bulb is basically a manual fuel pump of sorts.  It's purpose is to initially pressurize the fuel system and once the motor starts, the motor takes over using internal engine pressure and alternating vacuum inside the block to activate the fuel pump, sucking fuel from the tank.  With the motor running your primer bulb is no more than a fat piece of fuel hose so you can expect it to be soft at that point.


    From about mid 1987 on the new fuel pumps are different, but the old type were still used for the rope starter versions while the new type was used on the electric starter models. ???  The new type was moved to the LH side of the power head.  A 5/16" NF threaded hole was tapped into the original fuel pump vacuum location with a hose fitting allowing the hose to be rerouted behind the head to where the new pump is located just in front of the electric starter.  This new fuel pump was not designed specifically for this motor, as it was also being used on almost all of the larger motors of this date.   No one that I have talked to can explain why, except it may have been an engineer’s wet dream.   Maybe they had a quota of “improvements” that year?  And this engineer had never worked on one of these motors.

   My guess is that since the older small square fuel pump is very hard to repair (to keep the small parts in place on reassembly) so they developed this newer larger version, but did not do enough forethought in the mounting and found that it may be crammed in place on the right side, so they mounted it on a bracket on the left side.  However as any of you can see if you try to take it off, it is a bitch to access the attachment bolts.  Either fuel pump has to put out the same output to the carburetor, so either will work, again ???   Maybe it was a trial and the designation of electric start would be easier to make a distinction on a parts list?


   9.9hp / 15hp OMC fuel pump numbers taken from factory OMC wall chart

   9.9hp / 15hp rope start 1981-1992  =    old square type #393088
   9.9hp electric start 1987-1989 = newer elongated type #398514
   15hp electric start 1988-1989 =  newer elongated type #398514
   9.9hp / 15hp all 1990 >>>   =      newer elongated type #438616 (but apparently a different mounting)


Early small square fuel pumps mounted directly to the block for all up to 1987 & then rope start only until 1992 Later large fuel pump on electric start versions from 1987 to 1992 were mounted on a bracket


   Motor Not Revving Up Like it Used To :  If all other normal things have been checked out and the motor will not run at the faster speed, sometimes if it has sat for some time with the fuel not run out of the carburetor when it was put away, the main jet can get partially plugged.  The bowl of the carburetor can also accumulate some debris.  If this happens, the motor may start and run at a medium speed, but is not getting enough fuel to allow it to consistently run at a faster speed. 

    The other thing is as mentioned previously that for the pre 87s, the twist throttle gears may be out of mesh.

    Also if the timing plate linkage may have become disconnected from the upper throttle linkage peg, the motor will not run right.  This peg has a plastic bushing and wavy washer with a small cotter pin holding it all together.

    Or if your motor is the post 87 with the plastic topped carburetor, it may have a plastic sleeve on the cam roller.  This sleeved can become aged, break which will change your timing AND the throttle advance.

Broken cam roller sleeve


   Pre 1977 Upgrade Kit :  We who try to work on these older motors and who have not any inside information, grab at anything that pops up.  So listed below are a few tidbits of information, I have gathered thanks to my readers. 

    Here is some information gained that was supplied by a reader that purchased a kit off e-Bay .  The info from that site reads.   "New Carburetion Improvement Kit part# 387256 for Johnson/Evinrude outboard 15 HP.   There is not a lot of information on these kits but these motors were afflicted with a lot of idle, stalling problems and fouling of spark plugs.  The kit has a new cylinder head, new carburetor, arced throttle cam and all the gaskets and spark plugs."

    From what is shown in the photo below, I will make some observations.   The head is a post 1977 as evidenced by the scalloped recesses for the spark plugs and a different color possibly for identification purposes of this kit.   The spark plugs may be a hotter heat range.  The carburetor is probably just a 9.9 as if the 15 hp motors that this was targeted for may have been getting more of the bad fuel (because of a larger main-jet) than they could handle at a trolling speed.  And I doubt the head gasket was a little thicker, just a requirement for proper head replacement.


Carburetion Improvement Kit,  part # 387256

   One reader who apparently was a mechanic at that time e-mailed me.  Here is his comments 
 "In 1974 OMC put out a service bulletin, and "recalled" all 15 H.P. engines for that model year.  If you recall there was a gas shortage that fall.   The problem was simply put....bad gas, or at least fuel that was too low in octane, to run the "higher compression" 15 H.P.   So each dealer was shipped a kit for each motor sold....... It was the kit in your picture, and was in fact the head off a 9.9 HP model, with a different throttle cam. etc.   Owners were notified to return the motor for an "upgrade"  which for all intensive purposes was to turn them into 9.9's.  Some dealers used the kits on stock motors, and others didn't they just put them on the shelves waiting for a returned complaint motor.   A few customers motors were actually converted.   I think if you measure the head carefully you will note it's lower compression.  I just thought I'd shed a little, I was there and done that!   Richard C. Wilson".

    I can find no data in any of the parts lists to substantiate the above "higher compression for the 15hp", but can surmise that it may have been factory BS, where the different carburetor in the mentioned kit was indeed a 9.9 that OMC was trying for a while to appease the frustrated customers.  My reasoning is that if the fuel was indeed that bad, any extra supplied by the 15 carburetor would basically flood out or foul the plugs if trolled for any length of time.


    What I have found is that the head was also changed slightly internally to give better fuel flow at low RPM and as shown in the photo below identifiable by the scalloped out recesses for the spark plugs representing the heads found on the later 1977 and later heads. 



    From a old time mechanic that was doing warranty at that time, "If you examine the design of those powerheads, the fuel/air mix enters one side, is deflected up toward the head by the piston crown, then across the head and back down toward the exhaust ports.  Just like any crossflow 2 stroke.   However on the 9.9 and 15, the spark plugs are at a sharp angle, facing the fuel flow across the head.   So IF the fuel is not completely vaporized, any liquid gas splattering against the head runs right into the spark plug.  The redesigned head has little dams around the spark plug to try to prevent that.   It helped, but certainly didn't cure the problem".  


    I have not been able to pinpoint why the need for a different timing plate cam, however it is explained below.   It does make sense that bad fuel could have been one component of the problem.   But with the lower output of the straight magneto setup with points and condensers, the spark plugs could very well become fouled, especially at a trolling speed.


Replacement head mentioned above, Note the definite deflectors at the plug holes Here is the other side, Note the 8 bolt scalloped thermostat cover & the non painted head/primed cover indicative of the replacements

   Now the best information of all that put many of the pieces together came from John Gill who was an mechanic working at Big Bear Lake Lodge in the high country of California in the early 1970s when the 9.9/15hp OMCs came out.  The information below is his account of what OMC did to improve some problems with the early versions of those motors (1974-1976).

   When this series of motors came out, he attended the OMC school, and because the lake where he worked was purchasing these for their fishing fleet rentals boats, the 9.9/15hp were his specialty with his focus being on them while at the OMC schooling.

   John said the replacement head kit was basically for trolling, to reduce plug fouling, in BOTH the 9.9 and 15 magneto models.  It came in oxide red because it was not known if it would go on an Evinrude or Johnson, and was to be touch up painted by the dealer/owner using OMC touch up paint.  The same applies for the thermostat cover.

   These new head combustion chambers had the ridges/bumps around the plugs for deflecting the incoming fuel mixture away from the spark plugs to reduce fouling, but more importantly, the head was modified for an 8 bolt thermostat cover with revised cooling circulation to try and keep the lower cylinder from running cooler.

   It seems the original head and cover circulated the water in such a way that the lower cylinder ran cooler and therefore was more prone to fouling plugs at trolling speeds.  This is why a new thermostat cover came with the head.  Note the difference in the cooling holes on the cover gasket between a 7 bolt, and 8 bolt thermostat cover.  They are positioned, and sized differently to try and get more even heat distribution between the two cylinders.  The scalloped head and mating cover allowed easier insertion of a spark plug wrench, as the earlier ones were difficult to get a socket on.

   Then there was an issue with the Delrin magneto ring; John explained the later brown colored one was a tighter fit to make the magneto plate have less slop, especially at idle, so the pull of the flywheel magnets against the coil would not cause the timing to jump around as much and give smoother low speed running.  The modified throttle cam was to try and give more throttle opening when accelerating from idle so the engine would clear out quicker as it tended to load up with fuel/oil after long periods of trolling.  The revised ring also helped stabilize the magneto plate with the electric start option.


   At Big Bear Lake, the water is pretty cold, as it freezes over during the winter, is 80 feet deep, and is at 6800 ft.  There, even with the above updates, John said he always uses NGK B6-HS plugs instead of B7-HS, as the hotter plug tends to foul less than the regular Champion plugs.  That, and the lack of oxygen at high altitude, plus the much colder water all contribute to the engine running cooler, and therefore not getting the plugs hot enough to prevent fouling.  The last thing a boat rental marina needs is to go out and retrieve stranded boats with fouled plugs.  John related that the hotter plugs resolved this issue at Big Bear Lake.  However he suggested to run the normal plugs at sea level.

   Another thing to do at a higher altitude is to run the smaller prop, it lets the motor rev up into it’s power band with a full load in the boat such that it does not lug the motor down.

   Lastly, one winter he decided to put tune up kits in all the 9.9 hp fishing fleet engines only to have them all fail the next season.  It turns out the condensers all failed.  They were Taiwanese manufacture as OMC’s supplier (Phelon and Mallory) had shifted their sources to offshore.  John said he had fortunately kept all the old condensers and put them back in and had no more problems for years.  So he said "don’t replace the original condenser unless they go bad, and use a USA made one if possible".   He was from the old school and thought that at that time the electronic ignitions were far less reliable than points equipped version (1974-76).

   The great thing about John Gill is that he has had these 9.9/15hp engines in service since they came out, and always had a fleet of 30 boats with them. That, and every year, they would get some new year models as invariably, some got damaged in service, so he has seen every year model and version and knows them well.  He says the best were the first 3 years, as after that, plastic and electronics on the later versions tended to break.


   Manual Starter Anchor Bolt Threads Becoming Stripped :  If this happens the threads in the aluminum base plate are what is stripped.  The manual starter will become floppy and may have problems engaging the flywheel ring gear.  If used extensively in this condition, the plastic starter spool gears will get chewed up.  The only practical way to cure this other than replace the manifold, is to drill out the base hole and tap it to install a Heli-Coil insert thread back to the original 3/8" NC thread.


   Replacing the Manual Starter Rope or Spring :  The easiest way to replace the rope is to take the handle off, let the rope go to release spring tension and remove the recoil starter from the engine.  You can then remove the pulley and replace the rope.  On this recoil starter, the rope pulley is in two pieces held together with screws.   Watch how it comes apart and you should have not problem.   Re-wind the new rope (counter-clock wise) and re-install the starter.   Take a turn or two around the pulley with the rope to tighten up the spring a little and run the rope back out the rope hole and attach the handle.

   If you are just replacing the rope, use a little caution when you pull the pulley free of the rewind spring.   They stick sometimes and can pull the spring out.   You don't want the spring to come out of the lower metal housing if you can help it, otherwise it is a messy and tedious job rewinding the spring back into the housing.   Which brings us to replacing the spring, take the old one out, but be careful in that if it uncoils from the housing it can be dangerous, like a snake whipping around.  in replacing a new spring, they are encapsulated by a metal clip.   Some of them come with a metal clip retainer holding the spring wound up tightly.  Others the coil will not be wound as tight as needed to be placed inside the cup unit.  If the tight one, you are in luck, place the new spring in the old housing, then take off the clip.   Rewind the rope and reassemble as described above.


    If you need to rewind the old spring, or replace the large wound coil.  My experience is best to clean any accumulated grease out of the metal cup.  There should be a large flat washer UNDER the spring and on top of the inside of the cup.  To save you time now and or later I suggest you clean the washer and cup, then epoxy the washer INSIDE the cup aligning the hole in the spring with the cup's center hole.  Down the road, you will be glad you did as if you have to remove and then replace the starter unit, this washer can get slid out of position and you will find the starter will bind up.


   Start the spring into the cup base with the outer looped end sticking out the slot.  Then wind the spring inside of itself, being very careful to hold things tight as you wind each successive coil inside the others.  Once you get the whole spring encapsulated, you can take a brief breath.  Now you can wind the rope back onto the spool then locate the spool's inner peg into the eye of the spring.

   Do not grease this spring or the splines on the outer upper part of the starter spool.  The only oil I would use would be something light, like WD40 on the spring and nothing on the plastic spool or splines.  Plastic is somewhat self lubricating if there is no grit encountered.

   Occasionally you may encounter a rope starter where the pinion gear teeth may be chewed off a bit where they engage the flywheel gear teeth.  This will usually be found on electric start motors where the manual pinion gear did not retract down far enough to clear the flywheel gear teeth after the motor started.  This can be improved by loosening the center mounting bolt, raising it enough to allow the pinion gear to be lifted up and off the pulley's splines.  You will notice 4 splines on this pulley hub that mate with the internal splines of the pinion which do the actual raising/engaging of the pinion.  Pull the pinion up and off, rotate it and realign on another set of splines.


Exploded view of 1979 manual starter unit

    The manual starter spool assembly was changed after 1978.  The rewind spring was changed in that the inner end of the early springs had a sharp bend back end to attach to the spool, which proved a weak point and had a tendency to break.   In 1979 and later they added a separate starter spring retainer which linked between the spring and the spool.  At the same time the neutral safety interlock system and ratchet teeth were moved from the top to the side of the spool.    


   For reassembly, you need to have the anchor bolt backed almost all the way out of the housing (but not all the way, that is also why you epoxied the washer into the cup.  If you back it out more, the coiled spring may pop out of its housing when you place the unit in position.  If when you get the spool unit in place and the metal protrusion in the hole, but have problems aligning the bolt into the hole, hold the unit together as best you can so you can remove the bolt.  Now you can look down thru the bolt hole and align the hole in the housing to the threaded hole in the base.  The other larger hole washer needs to be installed under the anchor bolt's head, otherwise the starter gear will not stop on it's upward motion and bind in the flywheel gear teeth.


   You will find it harder to get the manual starter out and back into position if you are working on an electric start version, as this ring gear makes for a closer fit. 

    When removing the manual starter, you do not want the center bolt to come out, allowing the spring to come flying out at you.  The simple method of retaining this is to tape it up with electrical tape.


Here, shown on a 1991, is a simple way to secure the starter spool while doing any repairs that require it's removal  Here is another simple method, just use a 3/8" course nut on the bottom of the center bolt


LH side of a 1974 Evinrude showing the manual starter.  Notice the starter spool interlock ratchet teeth laying flat.

LH side of a 1980 Johnson showing the manual starter.  Now notice the periphery edge teeth starter spool interlock ratchet & red interlock dog.


   Removing the Flywheel :   If you have to remove the flywheel, you will first loosen the nut on top with a 7/8”socket wrench.  Unscrew this nut 3 or 4 complete turns so that it is even with or slightly above the top of the crankshaft threads.  You can usually remove this flywheel by lifting upward on the flywheel putting upward pressure, (even if it is mounted on a rack or boat transom) and sharply rapping the nut with a decent sized brass or lead head hammer so you do not damage the threads or the nut. 

   I usually have the motor on a stand or allowing it to set on the skeg on a floor, wrap my arm around the flywheel, lift up as much as I can and sharply rap the nut.  It won't work and can possibly do damage to the lower main bearing UNLESS you are applying upward pressure on the flywheel.  Remove the nut and you are there. 
   Most real mechanics don't recommend this method as it is not what the factory recommends, so I hesitate to post info that some purists may take offense to, but for me it works 95% of the time. 

   However of that fails, then you will have to use the 3 armed gear puller and the 3 holes in the top of the flywheel like it was designed to do.  Inspect the flywheel key for sloppiness or evidence that it does not fit snuggly, as this can allow the flywheel to move slightly & change the ignition timing.  Note, there is no lock washer under the nut here.  Be sure to replace the key in the crankshaft slot when you replace the flywheel.  The mating surfaces of the crankshaft taper and flywheel taper should be clean of debris and with no oil.


   If you are forgetful or the tom cat's late night haunts might be hiding the flywheel nut on you, a replacement is 9/16" x 18 TPI or National Fine.


Professionally made flywheel puller in position

Red Neck flywheel puller in position


   Flywheel Problems On Early Motors :
This applies only to the 1974-1976 magneto ignition motors.    We have here a situation where the point breaker cam sets on the bottom of the exposed upper crankshaft.  This cam is held in place by the bottom 1/3rd of the flywheel key.  This allows only the upper 2/3rds of the key to engage the flywheel.  I have seen it on (2) 1974 9.9s where the flywheel key repeatedly gets sheared off, but the rest of the key is intact inside the breaker cam.  Both of these flywheels had apparently gotten slightly loose over time and the tapered hole wallowed out slightly where it was mated to the crankshaft taper.  This left a slight ridge (approx. .006") on the upper inner edge of the flywheel hole.  Even if you tried to over tighten the flywheel nut, the flywheel could not be sucked down tight enough on the crankshaft taper to prevent future repeated key shearing.

   My cure was to use a medium sized 1/2 round file, carefully file off this upper ring (unworn metal) inside the flywheel hole.  This now allows the flywheel to be seated ever so slightly deeper and snugger on the crankshaft.  The later motors do not rely on this breaker cam and I suspect the flywheel hubs are made slightly longer to eliminate this problem.

   Ignition  1974 thru 1976 :  These motors have transitioned thru 4 electronic ignition systems. The first 3 years from 1974 thru 1976 has a straight magneto setup with points and condensers, but does have external coils, which are energized by what's called a "Driver Coil" located under the flywheel.  All OMC models that utilize points, will have them set at .020.  This point setting is usually stamped on the armature plate under the flywheel, and on the inside of the flywheel, so if you remove the flywheel you can see this setting. 

   For a complete worksheet on how to set, adjust the points on these magneto ignition motors CLICK HERE

   Ignition  1977 On : Then  from 1977 thru 1984 they have electronic capacitor discharge ignition, sometimes called a CD ignition, or as OMC calls it, Mag Flash ignition.  This has a charge coil under the flywheel that sends a small voltage to the power pack, which ups the voltage to 300 volts which is then sent to individual coils and then on to the spark plugs.  From 1985 thru 1988 a slightly improved version of the system was used that was called CDII.  Then from 1989 thru 1993 another system called the UFI (Under the Flywheel Ignition) was used, this incorporated the powerpack built into the electronic ignition that used a trigger sensor and it's all located under the flywheel.   This UFI can create another whole different trouble shooting system if parts of the system fail.

Below is a repainted 1974  rope start motor that had the starter, rectifier, the Neutral Safety lever & dog removed at some time.  No powerpack was needed nor used on these early magneto ignition motors.  This 1987 is a CD electric start version, notice the starter shaft at the top of the photo.  Also notice the terminal block on the rear & lack of the fuel pump on the starboard rear.
This 1987 is a CD rope start model.  Again the visible powerpack.  Also the shift handle is black plastic. 1992 electric start with UFI ignition, notice no powerpack on the block , but this one does not follow idea of the fuel pump being relocated as on most electric start versions


    It appears Bombardier now only offers a conversion kit to convert the 1989 - 1992 units back to the previous CDI system with the powerpack being mounted on the side of the head with only the trigger sensor and charge coil mounted under the flywheel.   It seems that during those 4 years, when OMC used the UFI with the powerpack, the charge coil and trigger sensor being integral, mounted as a sealed unit under the flywheel that the weak link was the trigger sensor.  I suspect this conversion could be a plus later down the road where replacement parts could be more readily available after the initial investment of the conversion.


   The rumor is that these UFI units were prone to fail oftener than the external powerpack units as they built up much more heat under the flywheel, causing some components to fail.   That said, I am still going to use my 92 as I have been for over 15 years until it does fail.

   Re-Assemble Timing Plate :   There are 4 parts that constitute what I refer to the timing plate/stator assembly.  First there is reference #41 or "timing plate retainer and link", next #46 is the "support plate" onto which is attached #47 bearing, ignition plate support" which is a plastic type large ring with a groove in the center.  The outer edges of this plate #46 accepts #47 bearing.   This bearing is made larger in diameter which has angled ends to be compressed and ride inside the #39 "timing plate and sleeve" unit that the stator is attached to.  However it will not just slide on.



    You will have to make a simple small wire staple that will hold the 2 ends in place while you slide #39 down over this #47 bearing.   The best I have found is to place the bearing so the joint is pointing straight to the rear of the motor.  Slide the timing plate assembly down which usually allows the staple to then fall out, if not you can reach in with an ice pick, dislodge it from the bearing.  Now it will fall on top of the block, so you can retrieve it with a small magnet.  The red arrow in the photo below is pointing to the staple.  The other bearing shown below is the position the normal bearing is extended. 


    One is brown (newer) while another is black, and oldest white version that did not put enough tension on the timing plate, which resulted in the timing plate being moved by the flywheel magnets at low speed, resulting in a bad idle.  This white version was probably made during the first couple of years production.  One OMC factory recommended that if the white or black was prevalent, then change the bearing to the newer brown one.


    This friction ring or "Bearing" as it is actually called has been modified to give a slightly tighter fit and when working on the older magneto ignition where any slight movement in the timing plate effects the point setting, it may be beneficial to replace it.  That Number is #322435.  To hold it in place while installing the upper timing plate, use a short  1/2" staple to retain it.  However you want to have this placed on the LH rear side so you can access it for removal.


    A Red-Necked repair of these loose bearings is to zap it in a Microwave and slightly open it up.  HOWEVER DO NOT OVERDO THE ZAPPING.

   This part has to be dry, no grease as it is really a friction bearing.  If you grease it, very likely the motor will (at a slower speed) and you not holding onto the throttle handle, it may change speed on it's own.


Shown below is #46 support plate with #47 bearing installed with another older black bearing shown below for illustration purposes Here you see the three versions, the white one being the earliest


   Power Pack :  The usual problem if the motor dies and refuses to start with no spark, may be traced to a bad power pack.  And this power pack can fail with no forewarning.   I have had an aftermarket one go bad after 75 hours of trolling time.   Apparently if you use the motor basically for trolling where the motor never really gets to turn up to a higher RPM, the power pack builds up internal heat, giving it more of a chance to break down internally sooner and short things out.   Therefore, if you use your motor strictly for trolling, be prepared and possibly acquire another power pack.   Cost of a new one is between $60.00 to $72.00, or you my pick a used one up for about $20.00.  However the service manual says there is no testing of the powerpack that will tell you it is good or not, but after testing everything else and it still does not start, replace the powerpack.


   I have a friend who stored his 9.9 in a heated building over the winter only to have the powerpack fail the next summer when he tried to start it.  The only thing I could come up with was that maybe it was on the verge of failing when he took the motor out of service,  just setting in a heated room allowed the internal components to break down and short out.  ??

   The electrical output plug will have a rubber boot with pins protruding for a male/female fit.  Depending on the year there will be 4 or 5 pins.  The center pin is the kill wire which is black with a yellow stripe.  This wire goes to the kill button.  I have ran into the situation where you may have gotten the wrong year of powerpack and had 5 pins on the powerpack side but only 4 pin receptacles on the timing plate /stator wires plug.  Here I drilled out the 4 pin unit in the center with a 3/32" drill to accept the 5th pin plug from the powerpack.   I then cut the powerpack wiring harness back just enough to solder another wire onto this black/yellow stripe wire, taped it back and ran that wire to connect to one wire of the kill button.  The other wire from the kill button is just grounded usually to the port motor mount and all the kill button does is ground out that wire.


   The power packs will be marked CD 2, with the 2 indicating the number of cylinders it is designed for.  On a 70hp 3 cylinder, it would be CD 3.  The V4s use 2 of the CD 2s.   OMC lists the same powerpacks for the 2 cylinder motors as being used for from 6hp to 55hp motors made from 1977 to 1982.   Aftermarket power packs may not have this coding on them.  When buying replacement powerpacks, you may encounter ones with internal Rev Limiters and of course at a higher cost.  Here it would usually be for a more high performance motor, possibly for the Sailmaster, where it would have a lesser pitch prop and designed for running at max RPM for a longer time under a heavy load.


   Coils :  The coils on this model, one for each cylinder, are bolted externally on the LH rear side of the motor block.  As you may suspect the top coil is for the top cylinder.  These coils have a orange and a black wire coming from them along with the spark plug wire.  The black wire is the ground.  The orange wires are the high tension wire.  The orange wires will have a dark stripe on them, with the top coil originally having a black stripe and the bottom one a blue stripe.  These wires go into a black rubber coupler that has 3 prongs in it.  The prong placement is identified by "A", "B" & "C".   The orange coil wires need to be in their respective locations in this coupler or the timing will be off.  The top coil wire needs to be in "B" socket which mates to the orange /black stripped wire in the other half of this coupler. The black wire "A" socket in this coupler is for the kill switch.  To remove both the coil assemblies, you may need to cut this black wire, (depending of it is a pre 1977) and then crimp on a "bullet type coupler" on each end for later continuity on reassembly.  The later ones have a connector.


   Spark Plugs :  Spark plugs for these motors need to be 14mm threads with a 1/2" reach.   They REQUIRE this thread length at 1/2".    They will have a 13/16" hex body.   The plugs recommended here will fit the right "reach" length.    If you try to put anything longer in, they will reach too far inward, hit the top of the piston.  This can possibly poke a hole in the piston when the motor starts, making for a complete tear-down to repair it.   

   To do a spark test, remove the plugs with a 13/16" wrench and see if you can get the spark to jump from the ignition wire to a source of ground across a gap of at least 1/2 of an inch.  When you are doing this, either leave the other wire on the spark plug or be sure to ground the ignition wires that you are not testing so that there is no damage to the CD modules.  It is even better if you can rig it so that you test both ignition wires at the same time.  Using this method, it is easier to notice if there is some erratic pattern to the spark.  There is a picture of a home-made tester on the "Trouble Shooting" article.


   It is not unheard of to have a bad new spark plug, but rare.  These motors if in good shape will run surprisingly well on only one cylinder, but will not have any power.  It is a lot cheaper and easier if you get the motor only running on one cylinder, (or even intermittent running on one cylinder) to swap out the plugs first thing on the agenda.


    The consensus from many users for many years is that the Champions plugs seem to work better on these motors, at least the newer motors using the electronic ignition.  Now that the Japanese plugs have made the scene, this has changed somewhat in relationship to the OLDER MAGNETO style OMC motors of this series (1974-1976) as they seem to run better using NGK BR6HS (a slightly hotter plug) especially if you use the motor for trolling a lot.


   The spark plug wires on some of these motors usually have an original aluminum or plastic tape on them with the recommended spark plug number.  Depending on the year, this tape may recommend different plug numbers.  If the recommended number is Champion L78V which will be the new number of 833M, this is the non-electrode "air gap" or "surface gap" plug that fires in 360 degrees to the sides of the plug.  Champion has changed the code system from what I was used to and now they do not really tell you which is a hotter plug, but they list a L76V or the new number of  827M, I assume this may be hotter than the L78V.     These plugs have an air gap in the insulator that is suppose to increase spark, but as the motor's ignition system gets older and somewhat weaker, the system just can't put out the voltage needed to effectively fire this type of  plug.  If you go out and run Wide Open Throttle all day, and just slow down when its required by law or you come in to the dock, then you should use these air gap plugs if your system is strong enough.  If you run slow using them, they will oil foul more easily as they have no idle bar.


   To check the recommended spark plug for your outboard, CLICK HERE.


Standard Electrode   L77JC4 Surface Gap L78V
The above illustrations came from Champion Spark Plug's website


   Then other models, at least the 1983 will have the tape recommending Champion L77J4 plug which would be the  corresponding conventional single electrode Champion plug.  You now may have to use  L77JC4,  or it's newer replacement #821 or the #821M for marine use.  If you can't get the "M" series, don't worry, as it is probably  just the outer metal coating that may be more resistant to salt air.  If you are using the motor to run to the fishing area and stop to fish or troll slowly for extended periods of time, these plugs are probably the better choice as they tend to not foul as bad as the air gap plugs do.   So, use the above information as to your usage to determine the plug you need, NOT necessarily the recommended one on the motor.

   When you see plug QL77J4 being recommended, this is the same plug but with a resistor or sometimes called a radio noise suppressor plug.  It would be recommended of you are running a VHF radio or sonar/depthfinder.  If all the above is confusing, YES, I agree, but it was all taken off factory information over the time-span of these motors trying to explain what they recommended at that time.


    The spark plug gap for the OMC 9.9 /15 is the same on all 74 thru 2000 recommended being set at .030.   But, depending on what book you look at, the recommended plugs and gap settings may vary.  This .030 plug gap works well for high speed running, but an old trick used by trolling fishermen is to re-gap them to .040 to help keep the plugs from fouling.  One book recommends Champion plugs for 74-76, being #L81C, while the 77-98 are QL82C, both which are hotter plugs, so maybe going to the next current hotter Champion plug would be  (older # QL77J5) or it's newer designation #874 or RL82C could help eliminate some oil fouling.   Champion plugs get hotter as the number increases.


   The NGK brand also makes good spark plugs, the recommended one for these motors being # B7HS (standard) BR7HS (resistor), or the newer Iridium series #BR7HIX, gapped at .030 will also work. However the common recommendation is to go up one hotter range plug #BR6HS for the weaker magneto ignition systems of the 74-76 motors. NGK plugs get hotter as the number declines. Their Iridium series has a smaller diameter electrode, which should make for a hotter and cleaner spark.

   Some home mechanics who frequent i-Boats have come up with another twist to help these pre 1977 motors out. "It is called "synching" or "timing" the spark plugs which helps too. You orient the ground electrode towards the intake ports when you torque the plugs, the tip of the ground electrode will then face the exhaust ports (RH side looking forward). This then allows the ground electrode to shield the tip of the plug from raw incoming fuel mist and prevent fouling which also gives better low speed trolling. Remember that the spark plugs are screwed into the head from the right side, leave the electrodes on the left (intake) side of the cylinder (on this small motor)".

   So here, especially on these 74-76 motors (because of weaker electronics) if you synchronize (or time) the spark plugs this helps too, as mentioned above.   Sometimes, you need to swap plugs to get them to line up at the right point when tightening, it's a trial and error thing. This alignment does not have to be exact, close may be OK. Sometimes, you need to swap plugs to get them to line up at the right point when tightening, it's a trial and error thing.

   If the above information is hard to grasp, maybe I can explain it in other words.  The best location for the spark plug would for the electrode (overarm ground attachment) to be directed rearward + - about 30 degrees.  This would allow for the open area for spark to jump INTO the cylinder, inward toward the center of the engine.

   It is hard and confusing to use clock rotation because if the plug’s position in the side of the head, but probably the gap opening at 3 o’clock, (pointing forward) would be that location, if that makes sense to you.

   Use a felt tip Sharpie pen to mark the plug's porcelain so you can see where to stop tightening it such that it is aligned as described above, and repeat for the other plug. So on this, mark the side of the plug to match the overarm, which you can then see from the rear when installed.   It also helps to lightly grease the plug threads/washer before starting. Sometimes, you need to swap plugs to get them to line up at the right point when tightening, it's a trial and error thing.  For those of you who do a lot of trolling, especially with magneto ignition motors, this can be a blessing in preventing fouled spark plugs.

   This synch trick came to me via 2-stroke motorcycle and Go Kart racers, it works for them. This is also how the new Evinrude E-Tec motor spark plugs are "Timed", but they have a injector diffuser pin that is the aiming point for the open section the the plug's gap.  However they advise to grease the threads to help facilitate this method.


   As a clarification, for the electronic ignition motors, most fishermen find the Champion L78V air gap plugs DO NOT perform best for their needs, while the  Champion L77JC4 plug have  a very good track record.

   Replace Spark Plugs or Clean Them ?
   What's the rule of thumb when it comes to replacing spark plugs ?  Run till there's a problem, or replace them once a year, every other year, etc. ?   There are lots of opinions on this one.  Spark plugs are cheap so some folks just replace them every year.  I personally clean, bead-blast them, check the gap and put them back in.  If I do replace them every 2-3 years, (depending on how much I run the motors) I save the old plugs for spares, sandblast them so they are a usable spare set on the boat but vacuum pack them to keep corrosion off.  If you do replace with new spark plugs, be sure you get the same configuration so that you do not have a problem at the start the next boating season.  

   Rectifier For Recharging if Battery of Electric Start Motors :  These motor's alternator produces 300 Volts AC current, to convert it to 12 Volts DC there is a rectifier installed in the line.  Leading from the power source under the stator / flywheel will be 2 sheathed cables.  One with orange wires goes to the powerpack, in that cable will also be a black kill button ground wire.  The other sheathed cable will have 3 yellow wires leading the the terminal block.   These will be 1 each of a yellow wire, 1 yellow with gray stripe and 1 yellow with blue stripe.  These need to be connected to the terminals of the terminal block to the same color wires going to the rectifier.   Coming from the rectifier is a red wire that again goes to the terminal block.  This red wire is then ran from the terminal block to the electric starter HOT side so it will charge the battery.


    You may find a rectifier that has another wire that is usually blue or possibly black.  This wire usually goes to the kill button on some versions of electronics.  These rectifiers are pretty universal on many sizes of OMC motors, where the boat is equipped with tachometer the yellow/grey wire usually goes to it.

If you hook up the wires to the battery backwards, you will more than likely ruin the rectifier.


voltage rectifier terminal block


   The photo below is off a 1992 electric start version, but the connections are the same from the start of this series in 1974.  This will be simplified as the fuel pump is different on the other side of the motor (connected by the black rubber tube) and it uses a different power pack under the flywheel, so does not obstruct wiring viewing in this photo.  This may be slightly different in that on this one, as I ran the start wire over the top of the block to the starter, where the factory usually runs theirs under and around the front of the block. 

   As shown, the #1 red wire comes from the starter switch and attached to the terminal block.   #2 wire attaches to #1 wire on the terminal block, runs down to the neutral safety switch.  #3 comes from the neutral safety switch and goes to the starter.  #4 smaller red wire comes from the rectifier to the terminal block, then forward to connect to the HOT wire from the battery on the starter switch.  This is the charging wire to the battery.
#3 comes from the neutral safety switch and goes to the starter.  #4 smaller red wire comes from the red to connect to the HOT wire from the
Wiring connection layout for electric start motor

   Installation of Hour Meter or Tachometer ;
The simplest way that I know of is by installing a unit that is called a Service Monitor.  They come in just an hour meter or a combo of hour meter and tachometer.  The hour meters sell for about $42 while the combo is $57 thru marine dealers.  On the back is the name SenDEC Corp. Fairport, NY  It is a small black plastic rectangle about 5/8" X1 3/4" X 3" long.
   It has a small LCD screen on the front.  It is a battery operated sealed unit, with a electrical wire about 20" long out of the side.  The wire is simply wrapped around a sparkplug wire, which apparently reads the impulse.  These can be used on either a rope or electric start motor whereas a standard tachometer needs a battery or electric start.  However if you are purchasing the one with the tachometer, you have to purchase models that match either the 2 cycle or the 4 stroke engines as the tach is not interchangeable here.
   The manufacturer's website is  Go on this website, click on Inductive Meters.  The SenDEC price is about $40, the Mercury brand unit is about $30 more.
Here, a rebuilt 6hp powerhead with  a hour meter attached on the thermostat cover.

   Throttle Block :  These motors are designed so that you can not manually start the motor (pull the rope) if the throttle is advanced much beyond the start position.  There is a dog (or interlock) lever on the top or side of the manual starter spool that drops down and engages in a ratchet that prevents the starter rope from being pulled.   These early (pre 79 motors) the ratchet notches for this neutral safety interlock are on the top of the starter spool, this interlock dog is activated by a linkage from the carburetor roller cam.


    Therefore it is wise to, each time you are going to start these motors to twist the throttle to the slow side, then bring it back to the start position.  The reason for this is that since there is some slack in the throttle pivot gears.  Many times, just bringing the throttle twist grip back from the fast position to the start position, the linkage has not reset it to the neutral safety position. 


    You can pull the starter rope handle right off the end or break the rope, if you jerk the rope hard enough and are not suspecting that the thing is locked in the safety position.


Manual starter for pre 1979 shown from the LH side, notice the ratchet notches on top of the spool Same motor as on the left but showing the throttle activated interlock dog that drops down into the ratchet notches stopping the manual starter from being pulled if twist throttle is in high speed.  Breather box is removed for clarity.

   On these pre 79s the rope rides on the LH side of the aluminum interlock housing (it is supposed to be there) then threads thru a aluminum bushing in the face of the front of the lower pan housing.  Some may think this rope needs to be routed otherwise, but if you try, it will jump from the spool or bind up if threaded under the interlock dog.


    Some may remove the interlock as shown in the RH photo below, then the rope is more in line with the bushing but not a necessity.

The black pivoting arm is the interlock Here the interlock has been removed.

    About 1979 the system was redesigned with the ratchets being moved to the outside of the spool and the interlock dog activator was moved from the throttle linkage to the timing plate.   On this later post 1979 units the dog spring's outer tail needs to lay in behind the upright steel retainer post on the rear edge of the starter housing as shown in the LH photo below.  


     I have seen some motors that the owners removed this interlock dog system, why I am not sure as it would be unlikely to want to restart the motor in anything above the start position.  Unless they took it apart, had a broken spring or could not figure out which way the spring went and left the whole unit off.   On the post 79 shown on the RH below, if you totally remove the red interlock it will allow the motor to be started in any throttle position.  For those so inclined, there is a screw partly under the outer edge of the timing plate that acts as a retainer/pivot for is interlock.  You might be able to rotate the timing plate enough to access the screw slot enough to unscrew it, if not then removal of the timing plate is required.


   Note on these later motors, the metal spool housing has the upright wire Bendix clip bracket on the front as compared to the earlier versions that has it on the rear.

Here is a post 1979 manual starter showing the ratchet notches on the outside & the red throttle interlock dog visible from the left side of the motor.  Note that the interlock spring goes rearward of the retainer post.  Here the motor is a post 1986 as evidenced by the throttle cable.  NOTE the interlock lever has been removed.


   Neutral Safety Position :   The electric start versions also have an electronic neutral safety switch, which is an inline switch between the actual starter button and the starter itself.  It is activated by the bottom of the spring detent plate that holds the shift lever in or out of gear. Occasionally if the motor will not start with the shift lever in neutral, check this switch.  The mounting bracket for this switch bolts to the side of the block below the fuel pump of the pre- 88 motors.  Loosen the bracket and the bolt holes are sloppy enough that you can usually get the switch button aligned so the bottom protrusion of the cam plate will push the button down when the shift lever is in neutral.

   Do Not Force Shifting Lever :  One thing to be aware of on these motors, is that the clutch dogs in the gearcase's transmission are rather large in relationship to some other models.  This is good in that you most likely will not experience the motor jumping out of gear because of worn clutch dogs if the linkage adjustment is not properly set.  But also at the same time, it means that you should NOT TRY TO FORCE the shift lever into a gear while the motor is NOT RUNNING.  You could very well break the shift lever, especially the newer plastic levers, or bend some of the linkage if the older metal lever.  If you HAVE to place the transmission in gear with the motor not running and the shift lever does not want to engage, try slowly pulling on the starter rope just enough to allow the clutch dogs to mesh in the gears. 


   Rev Limiter if in Neutral & Reverse :  On the timing plate there is a metal plate behind the actual throttle cam plate.  This is a stop lug that when the shifting lever is in neutral that the motor can not run at a high speed and possible ruin the motor, or if in reverse could create a safety issue.  This is essentially a Rev limiter.  It was not provided on the earlier motors especially probably pre 1977 that I have seen.

On this 1983, electric start motor with the shift lever in forward, the metal Rev limiter block is up stopping the movement of the plated metal timing plate.   This timing plate is rotated by linkage to the twist grip throttle handle.

On this 1987, manual start motor, note the shift lever in neutral & the black plastic Rev limiter block stopping the movement of the plated metal timing plate arm at the center of the picture just below the center of the flywheel. 


   Motor Makes More Noise Than Normal The 1st, and many times not as obvious, would be to be sure the insulation is still inside the motor hood cowling.  After that then look at the following things.  (2) Has someone has repaired the lower unit, like replacing the water pump, they may have left out the exhaust baffle (note that there is no baffle in the 74-76 motors).  This is discussed in a later section.  (3) Also it could be that the upper motor mount seal/grommet is deteriorated enough that it may be letting noise, (and oily residue) leak out.  This is a rubber seal that is located between the upper rear of the pivot shaft housing and the upper exhaust housing.  At this location on the exhaust housing on each side you will see a 1/4" Phillips head screw on each side.  These screws go into the upper rubber motor mount that is located inside the housing.  The grommet is the seal between the pivot shaft & the housing.  The above 2 conditions will have more of an exhaust type sound.

    If you have a tinny type noise or rattle, then it could be  (4) The pistons may be worn and could have lots of slop in the cylinders.  This however will usually quiet down as the motor warms up & the RPM is increased.  (5) The flywheel could be loose, but this will be more of a knocking type rattle.

Also look and see if the foam insulation is still attached to the inside of the cowling.

   Prop Spins While out of Gear Using Flushing Muffs :  When the engine is running in neutral using muffs, not in the water, the driveshaft will be spinning internally in the gearcase.  The driveshaft is direct from the motor and goes through the water pump and down into the gearcase where the transmission is.  The top or input gear is called the pinion gear, it engages both the forward and reverse transmission gears simultaneously, one turning RH while the other LH.   These gears are spinning all the time the motor is running.  The prop shaft supports the sliding splined shifter dog, which when centered between the moving gears, it is is neutral, then when this shifter dog is moved either forward or rearward engaging the rotating gears, this power is transmitted to the propshaft.  Since when running, there is always oil movement inside the gearbox the movement of the gear oil in the gearbox caused by these spinning gears which is totally surrounded the gear oil even though it is not in gear and as a result the prop may slightly spin.  This is perfectly normal.


   Special Conditions When Using Flushing Muffs :  The use of "muffs" in running outboard motors off the boat and not in a tank has become pretty universal.  However there are a few things to keep in mind.   Normal house water pressure is at 50# to even more, while the motor is designed to operate closer to 15# to maybe 20#.   You need to moderate the water flow into the motor, the easiest would be to NOT turn the water hose valve on full blast, you only need to get water to come out of the tell tail hole, then more may exit the exhaust thru the prop.

   If you turn it on full blast, you may blow out a internal motor water tube grommet, or at least force water past it and very likely will see water coming out of the juncture of lower motor mounts and the lower where there is a gap near the shifting rods.  This is the same as described under the "Trouble Shooting".

   You may also get water running out the upper blubber hole in the midsection exhaust housing, the lower unit and prop, simply because so much water is being blown upward inside the exhaust housing, since it can not go anywhere, it comes down and out wherever it can find an exit.

Also consider that until the motor has warmed up to 145 degrees until the thermostat opens, the only real exit out of the motor is the tell tale hole, thereby increasing the internal pressure more.


    Another thing is it that it is about impossible to adjust a motor trolling speed using muffs since there is no resistance on the propeller as you would have under normal on the water conditions.

   Water Not Coming Out of Overboard Water Indicator Hole :   Don't panic, but VERY SOON after firing it up, if your motor is 1977 or later and you get no water spraying out the "overboard water indicator " (tell tail hole or pee hole to most boaters), get a small single strand wire and poke it in from the outside of this hole while the motor is running.  There may just be slight salt water crystals, or if it has set for a while, maybe a bug crawled up there.  If this does not get water spraying, shut it off as you may have a defective water pump impeller, which, if left running without water into the motor, could ruin the motor rather quickly.  The reason I say 1977 or later is that the 1974-1976 did not have this tell tale as we now know it, which is explained below.

   If your engine overheats and this overboard water indicator is not spraying, shut it off, look for debris plugging the intake screens on the sides of the lower unit.  Start it up again and again look for water spraying out.  If you can NOT get water coming out this hole, then SHUT IT DOWN QUICKLY.  If you get steam coming out this hole, you DO NOT have water being pumped.  Do not let it run to where the engine block is so hot you can not place your hand on the top of the block without getting burned or the paint starts to scorch, as you WILL ruin the rings. 

    If there appears to be nothing plugging the water inlet holes on the lower unit, be careful as the screen behind these holes is merely plastic.  You have checked and cleaned the tell tale hole and there still is not water showing, then in all probability the impeller is aged, worn or damaged.  One thing that will wear the impeller more than anything else is to start and run the motor without any water going thru the pump for even a few seconds.  It needs the water flowing thru it to lubricate this rubber impeller.  If run dry, the ends of the impeller can get worn/melted off, and/or the nylon impeller housing can get melted, between the impeller rubber and melted nylon, plug the water pump outlet.  This chewed up rubber can then get forced up into the upper water supply tube where it enters the block possibly blocking flow, if this rubber debris makes it into the water jackets of the motor, again possibly making a blockage that is hard to remove without about a total teardown.

If you are running it in a barrel and get erratic water flow out this tell tale hole, you might consider pulling the motor out, then checking for debris plugging the water intake screens.  You may have to dump the water and start over with a cleaner tank.

   If all else fails to get water flowing, the one thing that will be the most likely need to replace is the water pump impeller. 

   For more detailed information on water pumps look at our "Lower Unit" section.    CLICK HERE

   Water Passage Route in the Motor :  As shown in this 1978 repair manual page shown in the LH photo below, the water is sucked into the lower unit thru the screened  holes in both sides of the gear case unit.  It goes up and into the water pump.  From there it is pumped up the supply tube to a block that is bolted to the underside of the power head.  From here it is directed into the bottom of the water jacket on the right hand side of the power head.  Some of the water can go out the overboard water indicator tube.   This overboard water indicator is commonly called the pee hole by many outboarders.  The water coming out of it will not be hot because it has not had time to circulate thru the block's water jackets.  It may be slightly warm but probably not enough for you to really tell if your put your hand in it.  It may be slightly warmer before the thermostat opens allowing more water to flow thru the motor.

   The majority of the water goes up in this water jacket, then back down and into the block.   Here it circulates around the sides of the cylinders, up and into the top of the head, thru both sides of the head and into the thermostat.  Then out the thermostat, (which should open at about 145 to 160 degrees F.)  up thru the water cover plate on the rear of the head, back out thru a passage in the head and out into the exhaust housing to be either sucked out thru the prop hub or thru the holes in the sides of the lower unit if the boat is not moving.

This diagram is for 1977 to 1987 with the thermostat on the bottom, later units with thermostat on top will be basically the same.   The 1974-1976 are the same except they do not have the overboard water indicator.

The 5 water inlet holes with slight grooves ahead of them are shown here with the blue arrow.  Behind these holes is the intake screen.  There are 6 round holes shown by the upper red arrow above the cavitation plate, but only 4 of them go all the way thru into the inner chamber & are for relieving back pressure if the motor is on a boat & is in neutral, otherwise the exhaust goes out the prop indicated by the bottom red arrow.

This powerhead shows where the Overboard Water Indicator elbow is attached to the water jacket.  This particular motor was off a 1974.  Since this motor originally had no indicator, the jacket was Heliarc welded here to match the later motors & tapped for a 1/8" pipe tap, then a plastic tube placed out the back lower cowling

   Once the water exits the block, it flows down inside the exhaust (mid section) housing, out thru holes in the sides of the lower unit and also out the prop exhaust.  There have been changes in this flow over the years.  The early units as shown in the photo below on the left used the square exhaust pipe.  The pre 1981  15 hp also used the square pipe. 

   Approximately 1981 saw usage of the round tuned exhaust pipe for all the later 15 hp motors.  And later nearing 1990 this tuned pipe was used on all 9.9 and 15 hp motors.   You will note that the early square pipes did not have a separate 3/8" copper tube exhausting the flow farther down inside the actual exhaust housing, but only let it spill out at the base of the motor inside the housing.

   As shown on the right of the photo below, the newer motors used this round, tuned exhaust pipe, they also have added a short 3/8" copper tube diverting the exhaust water farther down inside the exhaust housing.

   Not shown below is an last exhaust pipe, used starting approximately 1986 that was very close in appearance to the round long one but only about 6" in overall length.  It used the short 3/8" copper tube only, but about 1/2 the length shown on the RH one below.

   In the left photo below, notice the deteriorated rubber grommet protruding out the bottom of the exhaust pipe.   For an explanation of possible water tube grommets deteriorating where they attach to these exhaust tubes, possibly causing water blockage CLICK HERE for a link to my water circulation problems article.

Here is the early exhaust pipes from 1974-1976, these 2 holes on each side were apparently to relieve back pressure, or supply exhaust mist mixing with water out the rear hole?? Examples of both early & late exhaust pipes  with the long 3/8" copper tubes being the water inlet to the motor from the water pump


   No Overboard Water Indicator as we Know it, for the First 3 Years of Production :  The water simply was pumped thru the motor, then was exhausted out the bottom of the powerhead into a split section in the exhaust housing.   There is a rear chamber in the exhaust housing that is only open on the bottom, slightly below the one 3/8" blubber hole on the outer rear of the housing.   This system was common to all the earlier OMC engines prior to these years and was carried over into this series of motors thru 1976.  

   With this older system it is can be hard for the inexperienced to tell whether your water pump was really working until the thermostat opens allowing hot exhaust water out the blubber hole, as as compared to a motor that uses the newer tell tale system.   After the thermostat opens, you get lots of hot water and some exhaust gasses out this 3/8" blubber hole.   The one thing I have found however is that at an idle there is not much water blubbering out this hole.   If you are not certain, if the motor runs and the block does get so hot that you can not hold your hand on it for about 1/2 a minute, or the paint on the upper rear block starts to burn, the water pump is bad and needs to be  replaced.

   On the pre 77 motors the blubber hole exhaust chamber is open into the 6 holes on the rear sides of the lower unit, forcing the water out the blubber hole or being sucked out the lower holes when in the water.   Later motors used a exhaust baffle plate (with numerous holes in it) at the lower end of this chamber, this apparently allowed for better breathing of the newer motors since the overboard water indicator pulled water off at a different location.

   Back pressure from the exhaust inside the total housing forces some of the outlet water up which it pushes a combination of exhaust gasses and a watery mix out this upper indicator hole.   When running right and with the boat in the water, (which places back pressure on the exhaust) there should be considerable water mixed with some exhaust blowing out this hole.   If you however run it using muffs, you will get very little spray water out this same top exhaust relief hole.    Without exhaust back pressure, all the water that is exhausted goes out the bottom then out thru the prop.

   After the motor warms up, if you can lay your fingers on the flat place on the top of the block at idle RPMs and hold them there for about a minute without a great discomfort to them, (as the thermostat opens at about 150 degrees) if your engine is running about right.

   From looking at parts and service manuals for 1974 thru 1976 it is my observation that the "New Overboard Water Indicator" was not introduced in this series until possibly late 1976 or early 1977.  I have one 74 powerhead that has no provision for any overboard water indicator of any kind, but I have another with a model number suffix of G, it has a modified water jacket that has an 1/8" pipe fitting tapped into the upper section.  It also has some sort of a formed plastic spacer that goes over this section and the tapped Ell 1/8" pipe fitting going thru both.   This main water jacket area is thin and possibly the plastic was designed to help eliminate leakage.  This threaded hole is located in the upper mid part of the water jacket.  This location could only have been used for a manual start motor as it would have been in the way of the wire connection terminal block of an electric start version.  

   My assumption is that this is was NOT a factory modification, but an owner or dealer modification.  Then I have another same year motor with a prefix S, that has a raised threaded boss like the later versions on the lower section of the water jacket.  The simple method of converting would be to replace the outer water jacket with a newer one with the pipe thread outlet and drill a 1/2" hole in the rear of the lower cowling with a rubber tube connecting them.  The photo below shows the normal outlet position of this modified jacket.  All the 1977 motors that I have seen do have this Overboard Water Indicator.

   I would recommend anyone having one of these older motors without the "Pee Hole" to convert it to the newer style, as the is no doubt then if the water pump is working or not.
  However taking a page out of the newer motors design, it may be best to consider placing the outlet Ell nearer the top of the water jacket cover.   The reason here is threefold in that (#1) it is easier to rout the hose, plus (#2) it may eliminate a vapor lock in the upper water passage, that could slow down the initial water passage into the block.   And (#3) this sideplate CAN NOT be taken off the powerhead UNLESS the powerhead is removed from the midsection because you can not get to the lower front bolt under the rubber motor mount.

   This hole can be drilled and tapped at the upper location while this sideplate is still on the motor.  I would however place some chassis grease on the drill to pick up most of the chips when drilling.  Also do the same on the tap when tapping.   I would use a starter drill of about 3/16", then finish the hole with a R drill (.339 dia.) or a 11/32" (.344 dia.) drill and tap to a 1/8" pipe thread.  This material is thin, (less than 1/8" thick), there is a cavity behind it that gives enough room to drill and tap, BUT don't tap it deep (only enough to get a few threads in the jacket) as pipe threads are tapered and you need to have this fitting tight, pointing down and rearward when finished.   If you don't get it deep (large) enough, you can always tap it deeper.  It is rather hard to make it smaller if you find you made it to deep/large.

   Since pipe threads are tapered, tap it in only about 1/8"to start with and then try the fitting.  If you happen to go too deep it may bottom out inside and you would then have to shorten the Ell threads of the fitting.  A 1/8" pipe Ell with a 3/16" barbed hose fitting is what you need.  The OMC Ells #321886 are made of Nylon, and is recommended as they are more forgiving if you need to tighten more. Apply some non-hardening gasket sealer to the threads.  If you do need to tighten it in more, you may have to file off some of the metal or Nylon off the threaded screw boss just in front as the Ell barbed fitting may hit it while rotating and you can't get it rotated enough.

   One problem with this upper location is that you can not use this location if the motor is a electric start version because it would interfere with the electrical junction block from the power source, unless the upper junction block screw was removed & the junction block was rotated downward slightly & then only use the rear screw to retain this junction block.  Not really a good thing.

   In the photo on the left below, is an you can see the actual modified
Overboard Water Indicator using a OMC Ell.  The bright aluminum protrusion at the bottom of the cowling is simply a aluminum rod with a .085 dia. hole and the barbed end lathe turned to just have a snug fit inside.  The tubing is clear vinyl  3/16" ID, X 5/16" OD (just the same size as the hole is drilled into the cowling).  However I found that if there is a vapor lock inside the jacket, creating a blockage, this tubing can become so hot that it may rupture.  So I have now went to rubber windshield wiper tubing instead as seen in the RH photo below.  An O-Ring of a tight fit is slid onto the tubing to form a stop for the tube's protrusion out of the cowling.  The beauty of this is that IF the outlet hole becomes plugged, this tubing is flexible enough to be pulled upward out of the hole and easily cleaned.

   In the photos below you will see the differences between the early non tell tale hole and later water jackets.  On the right at the red arrow you can see the threaded hole for the water outlet Ell of the newer jacket.  You will also notice the lower right bolt hole, which is the one you can not get to without the powerhead removed.

Here is the actual conversion on  1974 Here is the water jacket off a 1977 & newer showing the fitting for the Ell outlet Here again is another 1974, but this one has a different sideplate with a hole location already there that only needed to be drilled out. 

This outward hole in the 1977 and later plastic overboard water indicator is small enough (.085 dia.) which is about the size of a wooden matchstick, small enough as to not draw off a high volume of water.
  From there, the water is routed up the rear of this jacket, over the top, down and into the block.  The flow then splits, then goes on each side of the cylinders and out the top into the water jacket of the head.  The flow splits again and circulates around the inside of the head’s water jacket, then out the bottom to the thermostat.  It then goes out the thermostat, when the proper temperature is reached, up to the top and then back down the thermostat water jacket, into the lower head, then into the lower block, out thru the exhaust housing’s divided section, out the baffle, then out thru the 4 holes on each rear side of the gear case above the cavitation plate.  These holes are there so that when the motor is being run in the water without being in gear, the backpressure is vented out thru these holes.  When the motor is in gear, the prop is turning and the boat is moving forward, this backpressure is sucked out thru the center of the prop.

   Even after the new overboard water indicator (tell tale) was implemented, the old vent mist holes were left in the upper housing.  They may have changed from one to two and also in size, but they still are there.   The later ones (1992) have a plastic cover that acts as somewhat as a muffler.   They are there yet probably as simply a relief hole so internal exhaust pressure will not build up inside the upper housing. 

 The 1974 overboard water exhaust indicator hole in the rear of the housing on this rebuilt & repainted motor, is the original water/exhaust hole that does not really indicate a lot as to whether the motor is working right. This 1983 motor is typical of the type of plastic overboard water indicator outlets you will encounter in 1977.  It has a small nipple in the center with a small hole (.085 dia.) in the center for the actual outlet.  Also note the smaller water/exhaust hole in the housing.

   This indicator water, when the motor is running should be not that much warmer than the water the boat is setting in, as it is just an indicator that the water pump is functioning and is exhausted from the engine before it has a chance to warm up.  So, if you
DO NOT see any water coming from this overboard water indicator hole, AND you are sure the hole is NOT PLUGGED, then you most likely do not have any water circulating in the motor. With the motor running for a period of time enough to warm it up and the engine is to hot to hold your hand on for any time on the block's top, there is NO WATER  going thru the motor.  If not, or a minimal amount of water is being pumped, you may see steam coming out the overboard water indicator hole. AGAIN, NOT A GOOD THING.  The engine should be warm enough that you can place your hand on the main block for a period of time without being unbearably hot.  If it is hot, or the paint starts to burn, it is too hot, shut it down immediately and hope you have not ruined the rings or scored the pistons/cylinder walls.

   Later in 1987 or so, the factory changed the the overboard water Ell by rotated it in 90 degrees more with the outlet pointing up.  The tube was made longer, re-routed up and around the outside of the powerpack and motor mount.  This greatly improves an access to this tube for checking if there is any question as to if the water pump is functioning.  With the tube in this position, you can, if careful, pull the tube off the outlet plastic fitting to get a better idea of where the obstruction is if there is one.  

   Many times the blockage will be at the outlet, but the other location would be at the Ell in the water jacket on the motor.   With the motor assembled in a running situation this Ell is about impossible to get at.  With this rerouted tubing, you can apply compressed air to the tube, blowing any internal debris away from the internal blockage area.   Sure this may not be the cure all, but it can give you an indication as to where the problem lies.  Heck you may even have to blow it out a few times before whatever was blocking the hole gets broken up and flushed out.  But this is a lot easier than burning up a motor, or having to tear the powerhead off just to clean a dirtball. 

This arrow points to the overboard water hole water tube rerouted up & around instead of under all powerpack & motor mount, making for better access

   It has been observed that before the motor gets warm and the thermostat opens, the overboard water indicator hole spews more of a volume of water than after the thermostat opens.
  Since the thermostat is near the last spot in the water cooling system, when the thermostat opens allowing a flow of the water to pass out of the motor.  This then lowers the pressure slightly at the overboard water indicator hole.

   I have come up with a new tell tale unit that makes a lot of sense.   Here I use the rubber grommet off a 1992 motor that fits in the lower cowling, I then lathe turn a aluminum or brass nozzle that just fits this grommet and extends out about 3/8".   I make the upper end the size that it just is a snug fit about 1/2" into the overboard exhaust hose.  This hose and nozzle is then just lightly pushed into the grommet with the nozzle being the pee hole exit.  In use, if the hole gets plugged with debris from the motors internal water jacket, all I have to do is pull this hose up and out of the grommet as indicated in the RH picture, pull the nozzle out of the hose, freeing any debris.  No wire poking in the hole that just pushes things back into the hose.   This is just held in place by friction at the grommet and on the nozzle from the hose.   You might have to replace the hose with a more flexible one if yours is hard.

   Note, DO NOT use light clear plastic tubing (sure it makes sharp bends easier, but) for the conduit for connecting to the water jacket to the nozzle.  I did this once and the water was hot enough that it heated the plastic enough that it swelled up, blew a hole in the now thinner walls, spraying hot water all over the engine.  Rather bad it you are running in salt water.

Modified Pee Hole Readily Removable Tube Nozzle


   Replacing the Thermostat :  It is advisable if you are having water pumping problems to also take a look at the thermostat, since if the water pump impeller had gotten any of the vanes worn off, this is a location that this chewed up vane rubber material can collect and plug things up.  If your motor is older, you may consider replacing the thermostat anyway.   The word is that the newer thermostats are constructed of a stainless steel, a better material if exposed to salt water and operate better than many of the older ones.

Standard thermostat used on many motors  

   The thermostats are supposed to open at about 160 degrees F.  which will allow the head to heat up just above the point that you will want to hold your hand on it for much more than maybe 10 seconds.  If it gets hot enough that you can smell burning paint, shut it off as the motor is too hot.

   On the motors from 1974 and before 1986 this thermostat is located in the lower portion of the outside rear water cover, which is bolted onto the rear of the head. The thermostat is in the very bottom of this cover. The cover can be removed from the motor without removal of the power head.  BUT you will have to remove the rear cover latch to allow enough room to access the lower RH cover bolt to be able to remove this cover. 

    This can be a little tricky in that you need to keep unscrewing the latch screw (1 flat at a time) from the inside, pull the latch lever rearward so that the screw is totally unscrewed allowing the lever to be retracted rearward out of it's hole.   Once you get it about 1/2 unscrewed you can hold onto the bolt head and unscrew the latch lever.  Now the inner actual latch can be pulled out after it falls off the screw, which then the screw is free allowing the thermostat cover to be removed.

    Now when getting the head ready to go back on.  INSTALL the two lower head bolts in the head otherwise you do not have enough room to get them in the head bolt holes if you start bolting the upper ones in first.

    The problem for reassembly of the cowling latch is usually that the wafer washer is hard to get aligned over the lever shaft AND then get the latch aligned with the flats coinciding with the flats on the shaft all at the same time.   I have found it easier to get the bolt and flat washer angled in place, then have someone with skinny fingers place the latch and wafer washer over the bolt while the bolt is then poked rearward but not out the hole.   At the same time push the lever/shaft in the hole, trying to align everything.   Rotate the lever as much as possible trying to get at least one thread started.   Don't worry about the placement of the latch until you get the bolt pretty well tightened, but still allowing clearance to slide it onto the flats.   Once you have the threads going, you can rotate the lever and at the same time tighten the bolt head with a wrench, speeding up the process.

   A Better Way   Make up a piece of 1/4 X 20 all-thread, screw it all the way in until it bottoms out, calculate how much more you need to secure the latch plus a nut of the same size.   It is a lot easier (but still a challenge) to start the nut on this stud than trying to align a bolt, wafer washer, latch and washer.   LocTite the threads into the latch lever, line up washer and latch on the inside, push the lever with the all-thread bolt in from outside and place a nut on the end of the threads, tighten the nut, and you have saved a lot of frustration.

   Before you get this far, you might just want to also install another piece of 1/4" X 20 all-thread in the head that holds the bottom RH cover plate.  Again for convenience and avoiding frustration as seen in the photo below, otherwise you also have to have the bottom RH bolt inserted into the thermostat cover before you slide it into place as there is not enough room to do it once the cover is in place. 

Here a early head, thermostat cover & latch are shown.  Note the studs installed as explained above.

   There was also a change in the head and the newer cover has scallops cut (as shown below in center) to allow better access to both spark plugs in about 1977, with the cover gasket also being changed at this time because the cover went from 7 bolts to 8 bolts holding it on.   This new head was changed for better fuel burning to help eliminate fouled plugs found on the 74-76 motors when trolling.  It had wishbone shaped wedges near the plugs in the combustion chambers.  This was supposedly for revised water circulation which was supposed to keep both cylinders more even in temperature and reduced plug fouling especially on the bottom plug.  The scalloped spark plug locations helped in installing the spark plugs.

The head showing the 7 bolt thermostat cover for motors from 1974 to 1976 The head showing the 8 bolt thermostat cover for motors from 1977 to 1986, note the scalloped cut outs for the plugs on this newer head The head for motors from 1987 to 1992 with the thermostat on the top

   The later motors, at about mid 1987 have a different head as mentioned and shown above.  The water jacket of the head that the thermostat is housed in, was changed to place the thermostat on the upper rear, making it a lot easier to access the thermostat.  This thermostat is all together different, in that is is encapsulated inside a sealed plastic unit.  And the thermostat plate gasket is simply a neoprene O-Ring.  When re-installing the thermostat on these, it might be wise to add some anti-seize to the cover threads.


   These newer motors from 1987 on when replacing the thermostat, there is a base seal under the stat.  This is a rubber ring bonded to a stainless steel base that when pressed into the head is so tight that if you get a stat kit with the seal, many times you can peel the rubber off the base and not even know that the base is still there.  The new seal will not fit because part of the old one is still in the head.   This seal is pressed in rather tight, so unless the rubber seal is bad, it is recommended not to remove it, but just take care of the thermostat.

As seen in the photos below, you can see the arrangement of these parts with the thermostat and spring in place, and with them removed.  In the RH photo below notice the bevel on the right (forward) end, which mates into a rubber seal pressed that is into the head.

The Thermostat cover removed Here the thermostat & spring are removed showing only the seal left in Here you see a side view of the late thermostat

   One suggestion, is that since these post 87 motors are so easy to remove the thermostat, is to occasionally remove it to check for debris that has accumulated inside it.  The photo of thermostat on the right above has some small fir needles and alder leaves that did not make it through.  And a dose of compressed air and flushing did not solve this one that was stuck open.  This one was so bad that after I installed a new thermostat, I dissected the old one, it had the small internal shaft seized so tight inside the outer tube that no way could it operate.  Maybe now I will occasionally pull the cover and squirts some WD-40 on this small shaft. 

    I have comparing both the old head bolt pattern for both the 74-76 and the 77-86 with newer 87 style heads.   The newer head with the top mounted thermostat could be interchanged with the older head.   If you make the conversion to the newer head, the coils may be slightly different configuration, but they can be made to function as the mounting bolts are all the same locations.  However you will also have to use the corresponding later upper cowling as with the post 87 head AND thermostat location, the old cowling does not have enough room to latch down.

    Do not run the motor without the thermostat.  The motor needs to come up to temperature in order for the spark plugs to to run hot enough to burn any excess mixed gas/oil so it will not foul, short out and make the motor run bad or die.   It also needs to burn the oil that may accumulate in the piston ring grooves so they do not become seized in the grooves which create more problems.  However if the motor is used in saltwater, be sure to flush it after usage as if there is salt buildup inside the motor, it WILL be at the thermostat area and if enough is there it will restrict the motor from cooling as the thermostat will become stuck shut blocking any flow of water as evidenced by the photos a bit farther down in this article.

Repainted 1974, NOT showing the thermostat, because it is buried in the bottom of the cover plate between the cover latch & the bottom of the head

1987, showing the newer rounded cover thermostat housing, retained by 3 bolts on top of the head


   Possible Water Circulation Problems :   One cause could possibly be is that if the motor has been run in very muddy or debris filled water, and that some of the internal water passages could be plugged.  Even if you happened to run aground on a sandbar.  If this is the case, the restriction or blockage will usually be in the RH water jacket sideplate, (the first water jacket the water is pumped thru).  However you can not take this plate off to check, clean the passages unless you remove the powerhead because the motor mount is in the way of ONE bolt underneath it.  The other area of restriction could be in the head at the thermostat.

   If you are absolutely sure that the water pump is good, and the passages are clear, and you are also as sure that what you are seeing is exhaust exiting from the thermostat area, the following could be the cause. 

   One thing you should do if no water is coming out, is to poke a wire or weed eater cord up in the overboard indicator to dislodge any debris there.  I have seen small bugs build nests in there.  If this does not do it, then possibly there is a blockage at the plastic elbow on the other end of the tube that goes into the water jacket on the motor.  The way things are put together, you can not get to the lower front bolt to remove this cover plate without complete powerhead removal.   

   For a link to the Water Circulation Article describing this in detail, CLICK HERE.   Go down to #6 in this article.    Another thing to look at is either the head gasket or the sealing area surfaces between the cylinder head and the block have become faulty. This would allow the combustion, the power from the explosion in a cylinder to escape across to the water jacket area, forcing its way down against the water pressure that's attempting to travel upwards. This results in a stalemate obviously and the engine overheats.  I'd suggest that you remove the cylinder head and inspect that area.

   The pictures below are from the 1974 motor described in Story #1 of my overview article.  Note -- in neither picture can you see clear thru the area on the bottom, and into the thermostat location.    This area has to be clear for the water to pass thru and cool the motor.   The water pump can be going it's job, but if the passages are blocked it is the same as if there water pump was not functioning.     


Head from the front or combustion area, showing blocked water jacket passages, behind the headgasket 

 Same head as shown at the left, but from rear, showing salt debris under the water/thermostat cover & the salt encrusted/plugged thermostat

   You may have some other problems other than the obvious of a bad water pump impeller, but the above listings are the most common.   For more information on water circulation problems CLICK HERE


   In the photo below, you will see the inner and outer exhaust water covers.  The main water enters the engine from the lower right blue arrow , the secondary from the upper blue arrow.  It also enters from the red arrows on the right to even the cooling.  From there it goes around the other passages in the outer cover, exiting at the green arrow hole along with the left hand red arrow holes.  Then it enters the block through a small hole in the water jacket which would be near the upper left of the inner cover.

   This motor had been "repaired" by some shade tree mechanic that most of the branches and leaves were missing, giving him a fried  brain.  These factory gaskets have a self sealant on them.  However this guy, (probably was trying to use the old gaskets) applied enough Permatex gasket sealer on both sides to 90% block three of these holes, completely block the other two, leaving only one open.

   This situation may well contribute to a poorly performing motor, becoming hot in one side of the block and restricting flow to the rest.

Here you see blocked water passage holes in the inner exhaust cover  

   Fuel Pump Repair/Replace :
  Typically this size of fuel pumps were used on about all the 2 cylinder engines in the same year range, from the 2.5hp up to the 55hp models.  The fuel pump is normally pretty trouble free device.  These pumps are activated by vacuum created below the pistons of the motor thru a hole into the inside the motor’s side plate that leads into the crankcase.  If a problem occurs, check the 2 hold down machine screws and the neoprene gasket under the fuel pump.  There is also a filter screen on the outside of the fuel pump.  

   The older pumps are basically the same as the new ones with the exception that the new bodies have 2 external tabs, that align with the diaphragm tabs and other gaskets to help make it easier to assemble the parts correctly.   The repair kits do not even list a kit before 1982.  You however can use the new kits in the older pumps if you are careful and replace the new parts just as the older parts came out.  Price on a rebuild kit is about $12.00.  There are 2 spring activated check valves, one on the suction and the other on the outlet.  There is also a small spring and plunger inside all this, making it slightly hard to keep everything in the proper locations while making the final assembly.  It is suggested that you use the 2 longer attaching screws as guides in from the back side to align things before the final assembly.  Do not be surprised that you may have to do back and reassemble it a time or two before you get it right.  One word of caution, do your repairs on a clean
LARGE uncluttered table as the parts are small which may tend to crawl off and hide.  Also read and understand the instructions that come with the kit.  For a link to the fuel pump article CLICK HERE.

    The newer fuel pumps are different, from about mid 1987 on, but the old type fuel pump appear to be still used for the rope starter versions while the new type was used on the electric starter models.  The new type was moved to the LH side of the power head.  A 5/16" NF threaded hole was tapped into the original fuel pump vacuum location with a hose fitting allowing the hose to be rerouted behind the head to where the new pump is located just in front of the electric starter.  This new fuel pump was not designed specifically for this motor, as it was also being used on almost all of the larger motors of this date and on.

1992, notice the new style fuel pump on left side, in front of the electric starter.  Note the additional upper flywheel gear for the electric starter.

Early 1987, with the old style fuel pump on the right side, but new style thermostat housing.  Black plastic shift lever. Manual starter flywheel.


   Fuel Tank Problems :  These engines use a single line fuel tank hose and a fuel pump that is a suction type, which has been used on OMC since 1959, therefore the vent on the top of the tank HAS to be cracked open.  The hose couplings are the same on the tank as on the motor, so be sure you attach the hose properly, allowing the fuel to flow toward the motor.  The primer bulb has 2 check valve balls inside of it making the gas flow only one direction and there are usually arrows on the bulb indicating the direction of flow.  The arrow needs to point toward the motor.  If you find where the primer bulb will not pump up and stay hard prior to starting, there is probably a air leak in the fuel line or fittings on the tank.   Also, if at  times and the requirement will be to pump the bulb occasionally to keep the motor running, this is usually an indication of a fuel line leak, as it is sucking air somewhere.  It can even be a split suction line inside the tank.   Assuming that there's no air leaks or restrictions between the primer bulb and the fuel supply, then the problem could be caused by faulty valves in the primer bulb. To check this out, with the engine running, hold the bulb in a vertical position, with the upper hose being the one that leads to the engine. This vertical position is best in order to have the internal valves work properly.  If that doesn't bring an improvement, it is strongly suggested purchasing a new primer bulb assy. 

   Another situation is when the boat and motor is setting for a while, if you leave your fuel line attached to both the motor and the portable tanks full, WITH THE TANK VENT CLOSED, that when the weather gets warm, the pressure inside the tank will force gas up into and past the carburetor float then into the engine.  This excess gas will then leak out and down the exhaust housing, mixing with carbon inside this housing which will cause a black oily residue to ooze out of the motor.

    There can at times be other problems related to heat and gas vaporizing in warm weather.  There is the possibility it could actually pop the internal ball valves out of one end or the other of the fuel connectors, plus possible internal damage to the fuel line bulb.  Some boaters when leaving the motor set for a while, will push the ball in with a pencil to relieve the internal pressure on the fuel line.  It is best to get into the habit of unplugging the fuel lines from both the tank and the motor when not being used.

    When the engine is running, the fuel primer bulb should NOT be firm, as when you primed it.  When the engine is running, the bulb is usually laying in somewhat of a horizontal position, the fuel simply flows through it (more or less about half full).  If you find that the bulb is hard, the fuel valve inside the bulb probably has come apart and the inner portion fallen out, turned itself around, and or then pulled back inside the valve housing.  In effect this is shutting off the fuel supply.  Also many times the primer bulb can, over time, get hard enough that it is hard to prime the engine  If this is so, it is probably about time to replace the whole fuel line assembly as the lines usually then also be getting hard.

    Some of the older Tempo plastic gas tanks with unitized fuel gage and fuel pickup with the rectangular clear plastic viewing dome, may WARP after a period of time and the neoprene gasket underneath does not seal anymore. If the tank is exposed to the elements, when it rains, the water runs in on the top of the tank around this seal where it collects.  This can be checked by simply tipping a full tank, if gasoline leaks out around the gauge/outlet, water can surely leak in.  As a layman's fix, you can usually remove the O-ring and use silicone sealer between the tank and this bolt on flange.

    In may metropolitan areas, disposal of contaminated gasoline cost upwards to $3.50  a gallon.  I think I could figure out a method of filtering, or straining it and then reusing it in a lawnmower or something.

    If the gas tank has been setting for some time with fuel in it and the fuel has somewhat evaporated or turned to a brown jellied mass, one method I have used to clean the internal parts of the tank, is to put some paint stripper along with some pea gravel or small rocks inside the tank.  Let it set for a day or two, then shake, twist and turn the tank repeatedly.  The rocks will help break things loose, where the paint thinner helps loosen the old gas.  You might have to do this numerous times changing the paint stripper, depending on the severity of the contamination.  When it is fairly clean, dump the contents and replace the pain stripper with rubbing alcohol.  Repeat until the tank in clean. The rubbing alcohol is cheap and will also mix with any water that may have remained in the tank, so the water is removed at the same time. The alcohol also evaporates, so you do not have a disposal problem, which you would if you used gas to clean the tank. 

   Fuel Octane Rating :   As for fuel, briefly the gas should be non-leaded, with a minimum of 87 octane rating and probably NOT ARCO brand, which usually is called gasohol.  This 10% alcohol additive may soften some of the older fuel lines, fuel pump diaphragm, carburetor gaskets and not be compatible with some current outboard motor needs.   The alcohol in the fuel will within a couple of months separate from the gasoline and will by it's nature attract water.  This is not good in a carburetor especially if it sets for a while.

    In a 1981 sales brochure it states,   " FUEL FACTS.  Unless you are a Johnson V-6 owner, you can use lead-free, leaded regular or gasohol in your Johnson Sea-Horse as long as the fuel is at least 86 octane.  Your V-6 requires a regular leaded or premium leaded, 89 octane gasoline."  
    I also have a 1991 owners manual that shows a MINIMUM octane rating of 67.  However I challenge those of you in most parts of the world to find anything under 85 octane fuel now days.    I suspect it is a misprint.

    And the 1991 manual also says "Use any regular unleaded, regular leaded, or premium unleaded automotive gasoline that has not been extended with alcohol.  Use of alcohol extended fuels is acceptable ONLY if the alcohol content does not exceed 10% ethanol by volume or 5% methanol with 5% co solvents by volume."

   This 1991 manual further states "The use of premium grade fuels is specifically encouraged.  Most national brand premium fuels contain detergent and dispersant ingredients advertised to reduce intake valve and fuel injector deposits.  These ingredients also remove and prevent carbon buildup on pistons and rings.  These ingredients can extend engine life while maintaining a high level of performance."
   I have repaired and run MANY of these motors on 87 octane fuel and have never felt handicapped or had hard starting because of it if everything else was right.  However I agree that 91 octane surely will not hurt the motor.  It also may improve performance if the motor itself has lower compression.  I know of one boater who never trolls but makes repeated runs over the same stretch and has tested both the 87 and 91 octane.  He says he gets a 20% increase in usage when using the 91 octane, which he has done the math and it is actually cheaper to operate.

   Fuel to Oil Mix Ratio :  The formula shown below will be on all modern 2 cycle oil sold in the last 30 years or so.  On most all of the pints and quart plastic bottles of TWC-3 oils there will be a narrow clear vertical window on the edge of the bottles.  On the sides of this window will have numbers representing ounces and Milliliters for other parts of the world.  Or you can purchase special measuring containers showing different ratios and the amount of ounces needed.

   DO NOT use oil designed for your weed whacker or chain saws as these air cooled motors run a lot hotter than the water cooled outboards, where the outboards require the specially designed TWC-3 oils.

    If your motor is to where you have to mix the oil, you need to know how much gasoline you will be needing, add the oil before you fill the fuel tank so that the oil mixes better that just dumping it in after you fuel up.  However if that is not possible, guess, add a majority of what you think it will take, fuel up and then add the rest matching the amount of gasoline you took on.  If you are using portable fuel thanks, before hand, when you fill it, add 1 gallon at a time, use a clean 3/8" or 1/2" dowel and mark the dowel at each gallon and use it as a measuring stick.  This way you can refill a partly full tank with the known amount of oil.

    A quart has 32 ounces of oil in it.  So for a portable 6 gallon tank to fill it using the 50:1 ratio would take 16 ounces, (1 pint) or 1/2 of the quart.


Gallons of Gasoline

Gasoline to Oil Ratio

Ounces of Oil to be Added

16:1 24:1 32:1 50:1 100:1
1 8 5 4 3 2
2 16 11 8 5 3
3 24 16 12 8 4
4 32 21 16 11 6
5 40 27 20 13 7
6 48 32 24 16 8


   The official OMC fuel/oil mix for most outboard motors in this year ranges has for years been 50:1.  This means 50 parts of regular gasoline to 1 part of outboard motor oil.  Remember that this was before unleaded gasoline or even possibly an octane rating that was listed at the pumps.  Now the recommended fuel is 87 octane gasoline.   Or before the outboard industry came up with any TWC oil, much less the latest which is TWC-3 outboard motor oil.  You probably will not see much of the older TWC-2 oil out there anymore. 

   You can spend your money and purchase OMC oil for about $6.50 a quart, or buy a name brand quart for $2.79.  Sure the manufacturers want you to buy their oil, and for a new motor under warranty, it may be advisable to do so.  But the consensus from many experienced boaters is to use any good brand of oil, as long as it has the TWC-3  rating, it meets or exceeds the manufacturers specifications.  The TWC-3 represents,  Two cycle, Water Cooled, type 3 formula.  The formula 3 has decarbonizing additives designed to be more compatible with the newer non-leaded fuel.

    So from the above information, you can see that fuel and oils have changed dramatically since these motors were first brought out in 1974.  In this instance, times have changed for the good, as we now have a better gasoline and way better outboard oils.

    Some dealers may try to tell you that new motor warranties are void if oils other than original manufacture brand oils are used and if a internal mechanical failure happens.  There was a court ruling on this several years ago.  This ruling was that for this requirement to be valid, the manufacturer/dealer MUST supply the required oil at no cost to the consumer.  The "law" basically says the manufacturer can't specify a BRAND NAME of oil, unless THEY provide the oil.  However manufacturers can specify certain grades, (TCW-3) etc. and deny warranty work IF those specific oils are not used.  It is very difficult to argue that cheaper certified TCW oils don't give long life -- there are too many motors running out there that say it does.


   Then, from 1985 to 1988, OMC actually recommended a 100:1 mix, this was written on a decal placed on motors of these years.  Observed on one 1985 9.9hp and the 1988 Evinrude 15hp shown in the photo below, there was a sticker on the powerhead cover next to the fuel line connection which shows cartoon icons of a fuel pump, and oil drop, showing a ratio of 100:1 for the fuel to oil ratio.

1988 15hp Evinrude showing 100:1 fuel ratio symbol

   Apparently there was some problems with some motor users (probably higher HP than what we are referring to here) so OMC issued a Technical Bulletin  #2162 dated March 1986, for the mechanics to remove the 100:1 decal, replace it with a 50:1 new decal and inform the owner, when the motor was serviced in an OMC repair shop.  This was recommended for rental, commercial and heavy duty service engines. 

    The factory apparently did replace some damaged power-heads because of this.  Apparently some motors, under the right (or wrong) conditions , if ran at a high RPM or under a load for extended periods of time, may seize due to lack of adequate internal lubrication.  There was another bulletin sent out in September of 1988 that kind of soft pedals around the issue, for a link to this bulletin #2211 CLICK HERE

    Back when OMC re-introduced their 50:1 ratio of oil, an old time marine mechanic was told by factory engineers that their motors would survive on 100:1 but they couldn't trust people to measure it accurately or control what kind or the amount of oil they used.   So they reverted back to 50:1 ratio as a safe haven.   The 100:1 was sufficient lubrication for most motors while running.   The bulletin had nothing to do with lubrication while running and everything to do with storage of the motor.  So the 50:1 you have a margin of safety in case of a lean carburetor on a multi carburetor motor or an overheat and it can still protect the clean internal parts while not used for a few months in a climate that can create internal sweating.   It appears that the manufacturer's lawyers were very careful to not create the situation where a possible class action lawsuit could not be presented, so they emphasized the storage internal lubrication theory where the average boater could not dispute it.

   Any lack of the proper oil ratio could very well be due to those larger boats that had internal fuel tanks where upon refilling, the owner may not have been accurately mixing the oil in a ratio to the fuel added.  The oil should be added to the tank before the fuel is added to ensure adequate mixing of the oil.  But on these built in larger tanks and an unknown amount of fuel needed prior to refueling it is very hard to be consistent from each refueling to another.  And I seriously doubt that the majority of the fuel gauges on boats really read right, or that the owners even know the tank capacity to start with.   In those cases, the only real way to be sure is to use 5 gallon cans and add the right amount of oil before you add the gasoline, then transfer into the boat's tank.

   On the 9.9/15hp we are dealing with here that use portable fuel tanks, it is recommended that you can use a wooden dowel marked off in gallons by adding 1 gallon at a time to the tank, then marking the dowel at that level, therefore you can accurately measure and can add the proper amount of oil to what fuel you add, there is usually no problem.

   Now with the TWC-3 outboard oils, the lubrication is way better than with the oils available even in 1988.  The OMC factory engineering/service department has said that the AVERAGE outboard will be used only about 15 hours a year according to one of their studies.  However there is documented usage of a 1988 6 hp Johnson purchased new and being used a estimated total of 1200 -1700 hours of trolling time using 100:1 mix.  He uses his 70 hp as a go to motor, then the 6 hp for trolling and this 6 hp was still running fine in 2010.

    The reason for mixing oil into the gasoline is that since these engines are 2 cycle, (sometimes called 2 stroke) there needs to be oil mixed into the gas to lubricate the internal parts (piston, crankshaft & bearings) of the engine, as there is no crankcase oil reservoir or oil pump as in the 4 stoke engines.

    Less oil than recommended in the fuel makes the engine run leaner and warmer (less power, causes detonation, overheating, but more important, could cause the motor to get so hot that it could seize the piston inside the cylinder walls), more oil makes the engine run rich and fowls plugs (less power too), forms carbon deposits and friction can increase caused by the carbon builds up, so more heat is also made.  Therefore a happy medium needs to be achieved.

   You will see considerable discussion from time to time of this fuel ratio mix on at least one of the major boating forums.  There are some that seem to be emphatic about sticking with antiquated factory recommendations even though the oil quality has dramatically increased during the last 30 years.  It seems that if you take a page from the new automobile manufacturers concerning recommended oil changes and apply it to outboard motors there should be a parallel where the old days it was recommended to change oil every 2500 or 3000 miles, now that has moved up to 5000 or 6000 miles.  This change pretty well has to do with better engine construction, the use of newer AND better oils


   Black or Gray Oil Coming Out of Upper Exhaust Vent &/or Lower Unit Holes :  Many of these motors are basically used as a secondary motor, and usually then as a trolling motor, which are ran for considerable length of time at a lower RPM.  The fuel oil mixture ratio of 50-1 is probably too rich for this type of usage. The black oil that seeps out of the motor is just unburned oil from the gas mixture.  The black color is coming from the carbon created during the pistons firing situation and is accumulated unburned inside the exhaust housing.  When the motor sets for a while the excess unburned oil in the fuel is mixing with and softening up the carbon, creating the black oozing oil.  You can reduce the amount somewhat by properly adjusting the carburetor idle jet to it's leanest position, also where the motor runs best at an idle. 

   This black or gray oozing oil could be also because of a stuck open thermostat which would not allow the motor to warm up.  However thermostats that get stuck usually do it in the closed position, so a bad thermostat is usually not the cause of this condition.

   Or, it could be a 6 gallon tank that the fuel line is still attached to the motor during storage while the outside ambient temperature is high enough to expand the fuel inside the tank to where the gas is forced into the carburetor and IF the needle valve is leaking, it will force fuel into the engine and out the exhaust which mixes with the exhaust carbon, then leaches out as BLACK oil out the prop's exhaust.  

   If you are getting a thick creamy gray oil oozing out the prop hub, it is essentially the same as above except the inside of the housing may have less carbon build up, therefore the color will be lighter.  
It is the same thing as described above, but possibly from a new or rebuilt motor that may not have been run much for some time and there is not a lot of carbon inside the exhaust housing.   If there is minimal carbon built up from previous runnings, the oil will be the thick creamy light gray color that has a lot of water mixed in it.   This could also be caused by the converging ring (#332395) is missing on the prop.  This plastic ring facilitates a suction from the outside of the prop when the motor is running at a slow speed to help suck more of the exhaust gasses away from the "thru the prop exhaust" when idling in neutral or running slow.

Here black oil is running out of the upper exhaust vent of a 1992  9.9hp after the motor was used for trolling using a 75-1 ratio semi-synthetic & then sat for 3 months Here a creamy colored oil is exiting the motor's exhaust housing.  This is basically the same concept as the black oil except probably in a cleaner motor, BUT notice the converging ring missing on this prop allowing a oil build up inside the housing

   This oil seepage may not be evident until a day or even a week after usage, then will seep out up to a couple of months later.  Most repair shops just recommend that for most boaters to then simply live with the idea that you may have to wipe it down after each usage. 

    You could also see this oil seeping out of the front lower unit water drain hole.

    It is observed that if these motors are used on a smaller boat as the only propulsion, and act as a "get there" motor and also as a trolling motor, this black residue is considerably less to almost non-existent because the motor gets hotter because of the faster RPM, where the motor has a chance to warm up, thereby "burning" some of this excess oil.

    Also if the engine was winterized with fogging oil then you would expect to see oil coming out of the exhaust because this would soften any carbony buildup in the exhaust system.   If it smells like gas  =  unburnt 2 stroke oil.   However if it smells of rotten eggs  = Lower unit oil.

    This many times may be confused with the lower unit oil leaking because of the color and location of the oil coming out.   Another reason as to this also happening is as explained above under the Converging Ring on the prop, is if this ring is missing it can hurt by not creating enough suction to pull the oily residue out of the exhaust housing at low speeds.

   Akin to the above situation, where spark plugs have continually become fouled, I have been experimenting with using 2 gas tanks for the last eight years now and for me, I am happy with the idea.  One, a 3 gal. tank, mixed at 100-1 with full synthetic oil and it is used  ONLY FOR TROLLING, the other is the 12 gal. main 50-1 tank also used for my 70hp outboard jet on the same boat.  Again, I am using the 100-1 synthetic only for trolling with the 9.9.  If and when I need to run with this 9.9 motor faster, I simply switch to the larger 50-1 tank.  However the winter of 2007/2008 in an emergency flooding situation, where I was rescuing a lady and her son who were stranded in her house with the water was still coming up fast, my main 70 hp jet water intake had become plugged with floating grass or hay, making it useless.  It was getting dark and we were caught is a strong very swift current pushing me broadside to a fence, I did not have time to change the fuel tanks as I was broadside to the current, with the 9.9 then my only source of power then.  I ran this motor at full throttle during the last half of this rescue mission for probably 20 minutes before I got to dry ground and offloaded the passengers.  As of 2013, I am still using this motor with no apparent harm.  But I was using the FULL SYNTHETIC oil mix.

   I got the above idea from the newer VRO oil injection motors.  This VRO abbreviation stands for Variable Ratio Oil injection.   The larger motors using it (not the 9.9 or 15hp however) electronically sense the different RPMs then automatically adjusted the oil injected ratio.   They put out a 50-1 ratio at high speed and a 100-1 at slow speeds.  Recently the motor manufacturers have gotten away from this VRO system and went to a direct constant ratio pump.  This apparently is because with the VRO on a large motor with multiple carburetors, if you are idling, or trolling at the lower ratio fuel mix, so you decide to pick up and run, the carburetors still has the bowl full of that low ratio mix, the engine will then be UNDER lubricated until fresh richer fuel is drawn into the carburetor.

    I have kept track of the run time at trolling with this 100-1 ratio with one particular motor over a four year period and can account for 43+ hours of only trolling, with only one a sputter, all it took to clear that up was to speed the motor up until the spark plug became un-fouled.  Then 4 hours later, back home I changed the plugs before the next run. 
 In 2009, I switched over to using a 75-1 ratio of a semi-synthetic oil mix with the idea of not having to worry about changing tanks, but I get the black oily residue as seen in the photo above, so possibly back to the 100-1 full synthetic ratio for all my running.

   You will see responses on some of the major outboard motor forums being very adamant to not deviate from the factory 50-1 ratio.  I realize that many of these knowledgeable persons, (many who may be old line mechanics) who may have seen blown engines in the past which may have been attributed to poor oiling.  However oils have changed dramatically even in the past 10 years.  Others on these forums expressing their concern may be just repeating what someone else has said.  And you have to consider that anyone who posts on a public forum does not want to stick their neck out, make a recommendation only to be chastised if the results turn out negative for whatever reason.  That is the way of life these days where liability's head pokes itself out.  That said, I have done the tests ON MY MOTORS for a number of years and you have read the results above.  I am happy with what I now use, but if your situation is different, then make your own decision.

    I many times use the boat in bays which are somewhat salt water, initially I used to switch tanks to the 50-1, when I use the muffs to flush out the trolling motor at the end of the day.  The thought here is to use the richer gas to leave a better oily film inside the motor when it is stored until next time.  I now think that the full synthetic oil is so much better, that I don't bother changing, however when flushing, under these conditions, I do NOT KILL the motor with the kill button, but choke it to kill it.  This will tend to also increase the fuel/oil inside the motor if I do let it set for a while because where I live, as I normally do not fog my engines for winter storage.


   Shifting Problems :  Any time you may have a shifting problem, do not immediately question the gearbox, ESPECIALLY if you happen to be using a motor that is hooked up to remote controls.  The cables can become seized internally, or possibly stretched, out of adjustment or even become unhooked from the motor.  There are adjustments of the cables at the throttle/shift control ends.


As a test, if you do have remote controls, disconnect them from the motor and try shifting the motor manually at the motor as a test as explained above

   Transmission Stuck in Neutral :   Do Not Force Shifting Lever IF Motor Not Running.  The above heading is wrong, but that is what many owners suspect.  One thing to be aware of on these motors, is that these transmissions (gearboxes) ARE NO SYCROMESHED LIKE VEHICLES.  The clutch dogs in the gearcase's transmission are rather large in relationship to some other models.  This is good in that you most likely will not experience the motor jumping out of gear because of worn clutch dogs if the linkage adjustment is not properly set.  But also at the same time, it means that you should NOT TRY TO FORCE the shifting lever into a gear while the motor is NOT RUNNING because of these clutch dogs possibly not aligning when the motor is not running.  While it is running at a slow speed, these dogs are designed to engage easily. Otherwise, you could very well break the shift lever, especially the newer plastic levers, or bend some of the linkage if the older metal lever.  If you experience this and you HAVE to place the transmission in gear with the motor not running where the shift lever does not want to engage, (as if the transmission is stuck) try slowly pulling on the starter rope just enough, and at the same time putting pressure on the shifting lever, to allow the clutch dogs to mesh into the gear recesses as they do become aligned.  Using this method, you find the spot you will be able to move the shift lever from forward / neutral and then into reverse easily.


     Any time you may have a shifting problem, do not immediately question the gearbox, ESPECIALLY if you happen to be using a motor that is hooked up to remote controls.  The cables can become seized internally, or possibly stretched, out of adjustment or even become unhooked from the motor.  There are adjustments of the cables at the throttle/shift control ends.


    As a test, if you do have remote controls, disconnect them from the motor and try shifting the motor manually at the motor as a test as explained above.


   Experiences of a Owner & Verification :  "Hi,   There is a lot of good information here.  A couple of things that might be of interest to you.

   I had a 15hp Evinrude that was purchased new around Nov 85 and was designated as a 86 model.  It had the plastic top carby which cracked along where low speed adjustment goes within about two years.   The motor performed well at full throttle but you had to use the choke at low speed otherwise it would stall.   I fixed it with epoxy glue and used a sealer when I reassembled it as the back corner as did warp.   It was interesting to read your article about the extra screw modification.

   This model had AC lighting, cable throttle however the thermostat was located at the bottom of the head.  The stop button was at the end of the handle, but did not have the kill switch.

   This was a 100:1 engine and was almost always run at this ratio.  The engine was used on the back of a 11ft boat and was run at full throttle most of the time. The oil was a mineral generic brand.   I put around 300 hours on this engine before it was sold and the engine was still running strong.

   The main issue I had with the engine was cracked swivel brackets.  They would crack about 1cm behind where it contacts the trim pin. The first one was covered under warrantee, but I had to replace the others myself as they said it was a result of abuse.   They were $95 Australian each at the time and I fitted them myself.

   I also had a issue with one of clamps stripping, which was probably due to over-tightening and the fact the engine was removed from the boat each time after use.

   The biggest disappointment with this engine was it top speed. A number of my friends had the same boat with Japanese 15hp engines.  All of them were a lot faster with one or two people aboard, however when you put 3 or more people in the boat the Evinrude would be faster.  I spent a lot of time experimenting with engine heights, trim angles fuel mixtures and even had a custom propeller made as the biggest one sold at the time was a 9x10.   In the end I gave up and bought a 25hp Evinrude which had no problem outrunning the Japanese 30hp engines of the time.

    The link below will show you what type of boat it was used on if you are interested.

   Cheers, Craig of Australia"


   Checking & or Changing Lower Unit Oil :  Here are two flat headed screwdriver slotted stainless plug screws, one in the front bottom of the right hand side of the lower unit (gear case) with the other above it by 6 to 7”.  The lowest is the fill plug while the top one is an indicator of when the unit is full. 

   There is another round headed #2 Phillips headed screw above and forward the lowest flat headed slotted lower drain plug screw,  DO NOT TAKE THIS PHILLIPS SCREW OUT.   This Phillips screw is the pivot for the internal shifting mechanism in the lower gearcase.  If you take this screw out, then the whole lower unit MAY have to be disassembled to locate, reassemble the parts internally using a special assembly tool.  However first I would try draining all the gearcase oil, lay the motor on it's side, use a stiff wire or ice pick, flashlight, prayer, beer and are VERY LUCKY while having a steady handed friend slowly moving the shifter lever to realign it so you can locate the yoke pivot hole, get the screw back in place with the inner end going into the shift yoke pivot hole.   If this does not work, then remove the lower unit so it can be shaken a bit to maybe get that shift fork pivot hole in line with the gearcase hole.


   This shifting yoke was initially installed as the first thing during the final assembly of the gearcase.  If you can not get it aligned by the above methods, then total removal of the shafts, gears, and the bearings may be the final alternative.  AND YOU WILL HAVE LEARNED SOMETHING, AND NEVER REMOVE THE WRONG SCREW AGAIN.


    In the photo below this lower unit has been repainted so both of the screw heads are painted, otherwise they are normally the silver color of stainless steel.  This pivot screw is indicated by the red arrow.  The fill plug screw by a blue arrow.

Note this prop has the converging ring in place mentioned above.


    The simplest way to add or change this oil is to purchase from an OMC / Bomardier dealer, a quart of gear oil in a plastic pump bottle.  The pump spout has a clear plastic tube with a threaded end, that screws into the hole that you will remove the lower screw from.


   Remove this lower screw, check the color of the oil that comes out.  If the oil is, a golden brown it usually is OK, if it is blackish then it is probably OK, but really needs to be changed.  If it shows no water contamination, then quickly screw the pump tube into the same hole.  You will not be able to stop some oil from draining out while doing this, but if you act quickly, you will not loose a lot.  Then remove the top screw so  it acts as a vent.  Now screw the proper threaded hose fitting into the bottom hole and pump or squeeze the oil in until it starts to run out the top.  Replace the top plug.  In replacing this plug first, you are now creating just enough of a vacuum on the oil that will allow you to unscrew the oil hose, install this lower plug before a lot of oil runs out.

    The manual says it takes 9 oz to refill an empty unit.  If you are just topping it off, you may not be able to add much before it runs out the top hole.  If  you have drained the oil and are replacing it, you think you did not get this amount in before oil flowed out the top hole, you may be right.  What has probably happened is a air bubble formed in the upper cavity, which pushed some oil out above it.  Usually you can let it sit for a while and the air bubble will dissipate, allowing you to add more oil.  If this does not help, then run the motor a bit in a barrel on on muffs, then try again.


    While you have the screws out you should inspect the nylon seals, as IF they do not make a secure seal, water can get into the gear case here also and the price for these seals are about $.50 each.

    If it is a creamy to light chocolate color that means there is water in the lower unit's gear case.  If this is the situation, then allow all of the oil to drain out.  Flush it out with Mineral Spirits.  It is suggested that you simply change oil, replace the seal washers under the screw heads and run it, but check it every so often to see how bad the leak is.  If it slight and you are NOT using it in saltwater, you can get by for some time or until the next winter (slow season)  by just removing the lower plug screw, allowing any water that may have collected in the section to drain out and then adding new oil.  


    If it continues to leak, be prepared to at least replace the prop shaft seals and the drive shaft seals under the water pump.  There is also a O-Ring seal around the shifting lever rod into the gear case itself.  You should catch the drained oil, inspect it to see if there are any metal filings of any kind in it, which could mean damaged gears or bearings.  These seals are a double, back to back seals, you can not replace the outer ones only, since normal removal requires a internal type slide hammer which will ruin the inner seal.  Therefore to replace the seals, the whole gearcase will need to be removed and disassembled.

   There is an option when replacing this gear oil.  You can replace the oil with regular gear oil, called Hi Vis by OMC, or now you can get synthetic gear oil.  The synthetic oil does provide a slipperier, more efficient oil that allows the gear case to run at a lower temperature.  This is usually not a big issue with these smaller motors, as they are usually being only used as trolling motors and/or as an emergency “get home motor”.  The word is do not mix regular gear oil with the synthetic oil.  If you intend to use synthetic oil, drain the gear case and refill it with the new oil.  Then make a note somewhere that you have installed synthetic oil in the gear case.

   The Propeller :   OMC factory props are normally 3 blades which available in different diameters and pitches.  They fit all of the 9.9 and 15hp 2 cycle motors from 1974, along with the,  8,  9.9 and 15hp 4 stroke 1995 amd newer.  They all have the 13 spline shaft size and are designed for the thru the hub exhaust.   The most often seen being a 9 1/2" X 10" for both the 9.9 and the 15 hp.  This means it is 9 1/2" in diameter and rotates one revolution in 10".    A  9 1/4" X 8" is shown for the Sailmaster, with a power prop is listed at a 10" X 5".   The Johnson Sailmaster  or the Evinrude Yatchtwin have extra long (25") shafts and are designed to push heavier sailboats into and out of moorage.


   The 1991 OMC parts manual lists the following aluminum 3 blade props  --   9" X 10",  9"X 11",   9 1/4" X 8",  9 1/2" X 10",  10" X 5",  10" X 7", & a stainless in 9 3/8" x 9".


   However aftermarket prop companies such as Michigan Propeller make other sizes also.  This lower unit can handle a prop up to 10" dia.   A 10" X 7" or a 10" X 5" seems to be a good compromise for a heavier boat, OR if the motor is used mainly for trolling, as it will not lug the motor down as much & let you go a little slower.   These slower pitched props work best on a heavy sailboat as they give more control and give a better bite for reverse.  


   If the aluminum prop blades get dinged by hitting underwater objects, and they are not damaged too bad, you try to remove the dents out by laying the blade against some heavy metal object, then lightly hammering the bends out.  Then you should file the blades edges down enough to remove any roughness or deep gouges.   New replacement aluminum props run about $50 each.


   If you loose power but the motor continuers to run, the rubber hub inside the prop that connects the prop to the drive splines may have becomes un-bonded and is slipping.  You might be able to turn the prop by hand and see if it slips, or you can remove it from the motor and with a permanent marker draw a line straight across through the center.  Put it back on and the next time you have the lack of power problem, remove the prop and see if the lines still match or not.  If they don't it is a sure sign that the hub has spun.  These can be replaced at a cost of about $50.

   Removing the Prop :  The prop shaft splines should be greased with marine wheel bearing  grease when reinstalling the prop, to ensure that the prop can be easily removed next time.  These props have a rubber mounted hub, so if the splines are seized tightly enough, when removing a prop, the hub can be pulled off or pulled partly off leaving the prop blades misaligned and out of balance.  The nut, a 13/16" (same size wrench as the spark plugs) needs to be snugged down so that one of the slots align with the cotter pin hole in the shaft, but not so tight that it has to have an impact wrench to remove it next time.  However a air impact wrench may be the only way to remove a stuck prop since it has a rubber mounted hub and any other method may ruin the rubber.  Be sure that you replace the cotter pin when you replace the prop.  The factory recommendation is to not reuse the old cotter pin, since it could be cracked and if reused may break and come off possibly allowing the nut to work loose, thereby loosing the prop.

   Converging Ring on Prop :  On the original OMC and some aftermarket props, the rear outer of the prop’s exhaust hub is a plastic ring with a scooped portion pointing forward.  This serves a couple of purposes.  One is that since this motor uses an exhaust thru the prop, it then uses this ring as a scoop to force water into this center exhaust hole, forcing the exhaust away, not allowing it to build up a backpressure inside the motor at a lower speed.   This action of pulling the exhaust away will also tend to suck any of the unburned oil that accumulates inside the exhaust housing, especially if you use it mostly for trolling and do not run the motor at a higher speed for long enough time to suck the unburned oil out.   Without this function, oily residue can show up draining out the prop for weeks after it is taken out of the water. 

Converging Ring

   The other thing it does is that when the motor is in reverse, it again helps relieve the possible exhaust pressure.

   Over time, these plastic rings can get brittle, then break off.  New ones are not expensive, about $10, and are replaced by simply laying the prop on a floor or workbench, then with a short section of 2x4, and a large hammer pound the ring onto the rear of the prop hub.  The prop has a slight groove that the ring snaps into to hold it in place.  You might consider doing this in a warm room so the ring has warmed up enough to not break it during you installation process.

   Remote Controls :  These motors were designed to be also used with remote controls.  But in purchasing these, you have to have three separate adapters and cable systems, (1)  the other for the  shifting lever unit (2) the other for the throttle system and (3) for the steering.


   First you will have to  acquire adapters to convert your tiller motor to be able to add the remote controls.   BUT, it is my observation that the post 86 to 92 motors seem to be lost in being able to connect a throttle cable.  If you go to , select you year and HP, on the bottom of the page is an accessory page link.  However it is woefully lacking factual data. 

   I suspect that the reason we see so little info on converting these to remote controls is that most of the motors rthat were fitted with remote controls  were sold by the dealers who then did these conversions after the sales and at that time they had better access to the factory kits.  And in these size of motors, probably less than 2% would be remotes.


 (1 & 2) There appears to have been 2 different OMC  "Remote Control Adapter Kits.   (1)   #386660 and (2)  # 398032.  These kits are only the adapters to the motor and did not include the cables or control lever box.  The throttle cable is ran thru the hole that normally is occupied by the kill button, then is attached to a peg located on the timing plate linkage rod.  On #398032, the kill button is moved over into the position under the RH front lower upper cowling where the starter button would have been.  The starter button is not be used in this case as if it was electric start, the starter switch would have been on the shift/throttle control box.   The #386660 kit is pictured below with it's illustrated installation sheet.  Here the kill button is moved to a new aluminum bracket that is bolted into the 2 existing holes on the front.
   The throttle cable was ran under the manual starter spool and attached to the peg on the vertical control shaft to the timing plate.  The twist grip handle was designed to be lifted into the up position, which  disengaged the cog teeth of the drive gears, allowing the cable to function without removing the twist grip unit.  This however creates problems when the motor is not operated with cables if the operator is unaware of the situation, as described later in this article. The angle that the tiller handle is mounted on the upper housing allows the handle to tip out and away from the motor for clearance.  If you snap the rubber cover off, raise the handle, you can see that the higher the handle is raised the farther disengaged the gears become as the throttle handle's twist shaft is not on the same plane as the intermediate shaft that the mating gear is on.

   The shifting cable was attached to the 2 plugged holes on the RH front side of the lower motor cowling.  And the cable end was attached to the upper hole in the shifter handle by a spring loaded twist pin.
   The control cables must be type (OS) that means the plastic ends are molded on, not bolted.

OMC #386660 Kit

             OMC #386660 Kit illustrations


   Also in the LH photo below you will see a quick disconnect clamp screw that attaches the older type pulley/cable steering system used before the more modern push/pull cable type became popular.  Here the center knurled knob us spring loaded and has  protruding pegs that insert in a double keyhole type slot for a quick disconnect of the motor without disrupting the cable.


    Shown below on the RH are the shifter split collar that goes in the location of the kill button with the push/pull cable inside it. 


OMC #386660 Kit installed for REMOTE CABLE steering Here the throttle cable parts are assembled, the upper snap is what connects onto the peg shown below


   The throttle and shifting control box with matching cables need to have the proper length cables to go along side the gunwale and bend to the motor BUT they need to be long enough so as to not bind when the motor is turned n all directions.

Here the throttle cable mounting bracket is attached to the motor & the tiller handle is removed Remote throttle cable connected to lower arm peg


   Factory outfitted remote motors usually have had a electric choke solenoid if they were in the mid 1980's or later.

    For the sail-boaters using these motors as auxiliary power where you may have to raise the tiller handle (which as   noted before disengages the twist grip throttle) there may be a couple of alternatives.   The Sailmaster version of these motors designed for sail boaters is simply a extra long (25") shaft.  You could purchase a remote shift/throttle control then mount it near the helm.  Also extending the kill button wiring to the helm area would be beneficial.  You would not really need the steerage as you already have a rudder.  You may not need the full blown remote kit as your need for shifting gears is not great, yet at times essential.  


 (3)   If you were to use the more modern push/pull steering cable system and attach it to the motor, there is a bracket available that attaches to the transom and is the attachment for the outer end of the cable.  The end of the cable would then be attached to the  front end of this #386660 mount.  Now a Note Here, for these older motors do not try to purchase original OMC steering wheel, gearbox and cables, as aftermarket product has far outstripped factory ideas 40 years ago. 


    There are a couple of aftermarket companies who specialize in these items, Teleflex for one.  CLICK HERE  The one used most on smaller boats is a rack and pinion gearbox that the steering wheel attaches to and with a cable running rearward to the motor.  The cable needs to be ordered in specific lengths (taking into account for running along the side of the boat and back out to the center) being attached to the motor.


    One thing to think about if you are ordering all your stuff online, is that in all likelihood, you are not familiar with terminology or understand completely what you are ordering.  So do not be surprised if IF you order a steering cable kit, you may not even get the steering wheel.   My suggestion is to go to a local marine dealer and have them set you up.  Time and cost in the long run my be cheaper.


Here is a photo snatched off Teleflex website


   Adjusting the Motor Pivot Tension :  If the motor does not stay where it is put when trolling, but vibrates and keeps moving from one side or the other, you may need to adjust the pivot shaft screw.  This is a screw that is mounted on the left hand side of the exhaust housing.  This screw has a coil spring under it’s head that you can not see.  By screwing this screw in, it tightens the tension on the motors pivot shaft.  Since it is located outside the midsection and if the motor is used in saltwater, this screw may become seized in the housing.  If this happens, avoid breaking it off in the housing at all costs.   I would, try to unscrew it, BUT BE CAREFUL that just the extended screw is what is turning.   A section in the trouble shooting repairs covers removing a seized bolt.   CLICK HERE


   Motor Mounting Clampscrew :   The 2 clampscrews that hold the motor to the transom WILL need service occasionally.  The pivoting handles of these screws should have oil placed on the pivot pins from time to time to keep them from seizing up and then breaking off.   To service these screws, it is best to remove them from the bracket.  There is a smaller screw that comes in from the rear and is screwed into the rear of the main clampscrew, holding on the large cupped washer in place.  Use a 7/16" socket wrench to remove this smaller screw, then you can unscrew the clampscrew and totally remove it from the front of the bracket, clean, grease, then replace.


    If the screw comes out hard enough to gall the threads in the bracket, you may need to run a 5/8 NC (National Course) tap in to clean up the threads. 

    Somewhere in production before 1983 the clampscrew handles were changed from aluminum to nylon.  The screw itself on the early models was made of aluminum.   You may want to keep these screws well oiled as replacement screw assemblies (the later screw is now stainless steel) cost $31.84 each, as of mid 2005.  In this area also later, about 1987 there were Zerk grease fittings installed in the front of the tilt hinge shaft boss, as shown in the photo above.


    Shown in the photo below are the motor clamp screw handles.  These are the later 1987 and newer.  The earlier ones were made of a aluminum, but I have seen 2 variations of them.  The overall length of all these handles were designed so that the ends have holes in them so that when the handles are rotated with both ends pointing inward a padlock can be inserted thru both handle ends.

Nylon clamp screw handles on a 1992 motor



   Reverse Thrust Kit :  I have heard about this, but never really knew what it was.  Many boaters thought it was heavier or different ratio reverse gears.  This seems impossible to achieve in the existing gearbox. 

   One day on my browsing e-Bay, I stumbled on the photo below.  Now if you go to one of the online marine parts sales ( for example, find the year and HP, then on the bottom of the page click onto accessories, then to miscellaneous, you will find "Reverse Thrust Kit" listed.  Here it shows the parts in the e-Bay photo below.

    In this photo, you can see the shallow water/reverse locking lever and link being welded together (as evidenced by the black/brown metal discoloration).  I originally assumed for more rigidity.  But after thinking about it and only being used on the sailboat version motors, this version is so the more will not tilt up when in full power reverse which could be needed in a sailboat auxiliary motor.   The Ell shaped part is listed as a shift stop lock.  Not having the lock in my hands, I am not really sure of how it really functions.


    Also in what you can read on the instruction sheet in the photo, in the lower LH of the instructions, it says "Drill 4 holes each here" with a line pointing to the 6 holes in the rear of the lower unit housing.   I can understand that in sustained reverse, you may need more water coming into the water pump as the direction of travel has changed in relationship to these existing holes.   The thing I don't really understand is of the 6 holes there, 4 are already drilled through to the inside.  Maybe they mean to drill out the other 4 blind holes (2 on each side) for better suction to the water pump in reverse.


   One thing in this online list, it lists this kit for "longshaft" motors, which I guess that they assume these will be used on heavier/deeper hulled boats requiring more reverse thrust.

   On my 1992 9.9hp motor, it has these bars welded together.  So maybe the factory, over time, decided it was better to stock and install only the welded unit for them all.


Reverse thrust kit


   Gear Type Throttle Linkage :  Motors made from 1974 to up until mid 1986 used this gear type linkage, which over time get worn and sloppy.  To remove the grip, there is a single screw on the forward underside #62.  You will also see a spring #61, upon reassembly everything needs to be in it's proper place or you cannot slide the handle far enough to the rear (UNDER spring tension) to be able to get the screw properly aligned and it is VERY EASY to get it cross threaded.

    If there becomes a lot of slop between the twist knob and the actual movement of the timing plate, it can be in numerous places.  But the easiest is to shim under the vertical plastic gear #109.  To get it out, the short gear pivot shaft #107 has the upper end treaded (can't remember the size), but if you reach inside the gear's center with a sheet metal or drywall screw, twist enough to have it grab in the plastic, you can pull the shaft straight up and out.

   Look at the gear closely when you get it out.  There will be a mark on one of it's teeth along with a mark on the mating gear tooth for alignment.

    I use old plastic butter tubs and make round washers that I slide UNDER this vertical gear, which pushes it up, engaging the horizontal gear deeper.

    If there is forward/aft slop in the linkage rod, you can wrap it at the clamp (#110) location with a few wraps of electrical tape to tighten that also.

   Also check for the snap ring #67 not being in place holding in the washer #64 onto the gear #65.

Twist grip & intermediate linkage for motors from 1974 to 1986 OMC 9.9/15hp

   One thing that could also add to looseness is that at some time in the past a "mechanic" may have greased the timing plate Nylon friction ring/bearing.  These need to be dry, and if greased the rotational pull of the flywheel will mandate CONSTANT control of the throttle/tiller IF there is sloppiness in the system.


   Has the Throttle Twist Grip Teeth Jumped a Tooth :  Check the twist grip to see that it has not jumped a tooth at the pivot point where the handle lifts up.  If this is the case, your throttle indicator for START will not be in the right location relationship to the carburetor and the timing plate cam.  This could cause the motor to not want to start when the throttle indicator is in the START position.  And if it does start it will not be able to run more than 1/2 throttle as the twist grip linkage will be off the one tooth.  


   AGAIN DO NOT RAISE the handle very high and twist the throttle at the same time for the 74-86 OMCs while you are running the motor unless you are prepared to disengage the gears, and change the twist grip timing relationship.  This is explained in a subsequent section.  Later, in about 1986 a throttle cable push/pull system was used to eliminate this problem. in about 1986 a throttle cable push/pull system was used to eliminate this problem.


    You can check this by raising the handle, looking from the front at the mesh of the teeth on the 2 gears.    One has a WIDE tooth that has to mesh with the other gear that is missing a tooth.   If these teeth get out of mesh, about the easiest way to re-position them is to tip the handle as far rearward as possible, use a medium sized screwdriver, position the twist grip so it is aligned as best you can get it, then insert the screwdriver between the teeth, pry one tooth past the other and hope you do not break one off.  The other method is to pull the twist grip off, tip the lever rearward, slide the shaft forward, realign the teeth and reinstall the retainer screw in the twist grip.


    The nut seen here is the #83 nut mentioned in the #6 section above.

Gears in mesh Gears out of mesh



   Not Enough Tension on Tiller Handle :   For the early motors up to about 1986, if the tiller handle will not stay where you put it vertically, you will need to tighten it.  However this is not done as you would at first suspect.  The 3/8” nut showing at the hinge, when the handle is raised, is only a lock nut.   It has to be loosened, and the bolt's head end, that it is accessible from under the steering bracket needs to be tightened, then this visible lock nut retightened.  It is however recommended that you remove the bolt, clean the nylon bushing #83, then apply a slight amount of grease before you reassemble it.  If you do take this bolt out, be careful that you re-engage the throttle shaft dog teeth properly when you reinstall it.

   Cable Type Throttle Linkage :  Motors after about 1986 eliminated the gear disengage problem by going to a cable throttle linkage system instead.  The photo below shows a 1992 with the kill button on the side near the twist grip.  The earlier kill buttons were on the end of the twist knob, but the cable system was the same.

If this twist handle seems to not want to move, the cable may be dry and binding inside, or the timing plate may also be dry and dragging.  To test this, remove the ball socket end of the cable at the timing plate, now see if the handle will now twist.  If not, then the problem is in the cable.  You may be able to remove the cable, then get some penetrating oil in to lubricate it.


   To disassemble this unit, you will notice a small round hole (about 3/16" dia. ) at the arrow.  There is another 180 degrees and on the bottom.  You need to use a couple of punches, depress both of these at the same time, while pulling the handle off front-wise.  A helper can be useful in this instance.  Inside this plastic is a dual rotating cam slot which a center cross-pin enters into spacers, and rollers (illustration # 3, 18 and 20 as seen below) which is in turn attached into the cable end.  When disassembling this handle, be careful to not loose these spacers and rollers.   With the center pin removed, this cable needs to be pulled outward through the handle.


   When twisting the handle, the cable moves in or out.  This outer cable rear end is then secured by a pivoting ball in the lower cowling.  The rear inner cable has a screw on adjustable end attached to a small ball socket on the timing plate.  You can make some adjustment in this linkage by moving the threaded ball on the outer threaded cable, and or at the cable rear end.  If you are replacing the cable, be sure to set the ball and cable end at very close to what the original cable settings were, otherwise you may never get it set right, using a guessing game.


To disassemble, push in on the 2 plungers simultaneously  (top & bottom) & pull the twist handle forward.  This reveals the slider cams.


   There is a black plastic twist knob on the end of the twist handle.  This is a slow speed idle stop, (NOT A CARBURETOR IDLE ADJUSTMENT).

    This can be tricky to get set UNLESS the cable is adjusted properly on the low end of the speed setting.  And many times not worth the effort.

Exploded tiller handle for cable throttle control system from 1986
To access the parts list for the above illustration CLICK HERE

   One thing you may look for in the cable system, is that on the end of the cable where the nylon socket is that attaches to the ball, is this cable end seems to rotate slightly depending on the position of the handle's height.  The nylon socket is threaded onto the end of the cable's shaft.  You may have to thread it on farther or less to position it so the socket is about level with the handle elevated slightly, otherwise if it twists far enough it may pop off the ball.


   If this cable is not adjusted right you might have hard starting when putting the twist grip at "START".  Incorporated in the end of these twist grip handles is a slow speed trolling type knob that can be rotated to supposedly a slow speed stop.  I have found that this slow speed knob many times does not work, unless you do adjust the cable end also.  To set the cable end, I would put the shifter in forward, then move the twist grip to where it will not go faster.  With the cowling off, you can see the neutral safety block of the shifter unit blocking the timing plate from further rotation.  I would use this stopped setting as a point to readjust the cable end so that the "START" position on the twist grip would now align with the pointer.  The rear end of the cable just snaps over a ball on the timing plate linkage.  Snap it down and off, twist it on the shaft, then try to see if things are better.   You may have to do it a few times to see just where things line up.   Make an adjustment and the twist the grip to see what changes.

   You can now adjust the slow speed knob, (on the end of the twist grip) as your slow speed stop, after you have played with the idle knob, but in conjunction this slow speed knob. If you adjust one, you may have to readjust the other if at the SLOW end of the running.


    Occasionally you may encounter one of these cable systems that the twist grip will not stay where you put it.  The motor will usually slowly slow the speed down by itself if you let go of the twist grip throttle.  A solution for this was passed on by a retired marine mechanic.  His solution was to take a electricians #6 copper grounding split bolt clamp, then simply clamp it around the throttle cable in an out of the way place.  There is enough room on the underside the the tiller handle.  Tighten the nut enough to just squeeze the cable inside the plastic sheath.   Then tape up where you split the loom with black electricians tape.  Works like a charm.


#6 copper grounding clamp squeezing the cable on a 1992 motor, placed under the tiller handle & taped for exposure. #4 copper grounding clamp squeezing the cable on a 1994 motor, inside the lower cover

    At this time OMC added the lanyard type kill button on the end of the twist throttle handle.  This created a problem if you used the extended universal joint tiller handle, as you could not reach the kill button with this extended handle installed.   A year or 2 later the handle was changed slightly and the kill button was moved to about mid handle left side.    This button is also a man overboard kill switch.  The motor comes with a red coiled plastic cord that is attached to a split plastic section that is inserted UNDER this button.  The other end of the cord is to be attached to the operators wrist, so that in the event he falls overboard, this insert is pulled out and the red button goes deeper in the housing, the connection is disconnected, just like turning a switch off.   In use this red button HAS to have either the lanyard insert under the button, (turning the ignition ON) or a another smaller that one called a restart clip that is inserted there if the owner decides he does not want the lanyard.


   Zinc Anodes :  On motors from about 1983 on you may see (2) 1/4" holes drilled on one side of the cavitation plate.  These are for attachment of a zinc anode.  These anodes are threaded and bolts come up from the bottom with the anode on top of the cavitation plate. These holes are evident in the picture below of the prop shaft being removed.

    Removing Twisted Off bolts :  Here you MAY have some choices, depending on where it is broken, and if there is part of it protruding enough to get Vise-Grips onto etc.  The other option is to just leave it broken off and go with one less bolt IF that is all that is broken, if more than one then you will have to decide if it will create a functional problem.  Many times the stainless bolts are stuck worse than regular steel bolts, because of the oxidation between the two different metals.  And stainless is a lot harder to drill than mild steel bolts.  It will be a white powder.  I have not had any luck using an easy out in any of these instances.


 (1) If when you try to unscrew the bolt, it has resistance to where you feel it may twist off, (remember these will usually be 1/4" bolts) play it careful.  Usually if you try to use penetrating oil in this case the oil will not get UNDER the bolt head.  The corrosion will normally be between the bolt shaft and the metal surrounding it and not in the threaded area that it is threaded into.  My solution here is to use a small  angle head grinder, remove the bolt heads.  Now you should be able to pry the cover plate off these now bolt studs.


   Now use method #2 to remove what is still in the base material.


 (2) If you have broken it off, and it is protruding enough to get ahold of it, (1/4"), I would first soak the area with penetrating oil, If you do not have any, the best alternative is hydraulic brake fluid.  Build a dam around the broken bolt if possible with modeling clay.  Let it set for a few days, then during this time, tap the protruding end with a hammer a few times.  This vibration tends to break the bond and allows the oil to soak further in.  Now try a Vise-Grip pliers to see if it will turn.  If it does, only move it slightly, then back up, then try to loosen it again.  If you do it all at once, you can break it off again, and this time flush with the housing.  OK, this is the easy one if you got it out.  If not, then you might try to heat the aluminum up slightly with a torch in the area of the broken bolt.  Don't get it overly hot as you may melt the aluminum.  What this may do is about the same as tapping it with the hammer in that the aluminum will expand faster than the steel bolt, which may help break any bond between the two.  Do the soak and tap again.


 (3) If the above failed or the broken bolt is flush with the housing, I would again use the soak and tap/heat method, but now you have to drill it out as close to the exact center of the broken bolt as possible.  If it is a 1/4" bolt the hole size on the inside of the threads is about 3/16".  I would use a center punch, try to locate as near as possible the center of the bolt.  Remember that this small bolt the threads being a spiral, that you will see the outside of the threads, while on the other side you will be seeing the female part of the casting as looking straight down on it.  Try to locate the center where there are no threads.  Tap the center punch lightly.  Look to ascertain if it is near where you want it.  If not then place the center punch back in the original hole, then angle the head end away from where you want the hole to really be.  Tap it again at this angle and you may shift the original hole slightly.  When satisfied that is is as close as you can get it then clamp it in a large drill press or milling machine, so you can clamp the part rigidly so you can control the drilling.  If you do not clamp it securely to the drilling unit, the part WILL shift and you will drill off to the side.  DO NOT DRILL IT BY A HAND DRILL WITH NO RIGID SUPPORT.  Use a GOOD sharp drill bit with the hopes it will drill straight and not run out to the side.  You can drill it with a hand power drill motor, BUT the odds are that you will not be able to hold it straight and the bit will wonder.  Use a cutting oil on the drill as you are drilling.  If you succeed in having it centered, the bit will go hard, until it breaks out thru the end of the bolt inside the hole.  You may be able to feel the difference it at this point, as it will break into a hollow space below the broken bolt and into a deeper threaded section of the casting, as the threads will always be slightly deeper than the bolt. 

   If you did not get the drill lined up exact or it shifted so the drill took out just part (1/4) of the threads on one side, this should be no problem as there should be enough to still hold.  Now you can use an ice pick or cape chisel to get under one side of what is left of the threads & work them out, and possibly use a Dremel tool with a carbide dental burr to cut the bolt shell in enough pieces to extract them from the threaded hole.  This will free the hole enough to run a tap into it & clean up the threads.


 (4) If the above #3 failed and the drill ran out to one side, you will have to enlarge the hole enough to install a Helicoil.  This is simply a somewhat square wire tightly wound as a spring in the exact thread pitch as the desired new threads.  You will need to purchase a Helicoil kit, which includes  the proper drill size, oversize tap and an installation tool.  Here again it may be best to use a rigid drill press or milling machine, clamp the part to ensure you get it close enough to align the new threads to the other mating surface.  Here you might use a appropriate undersize end mill to clean up the butchered hole to remove what is left of the broken bolt before you use the Helicoil drill.   Follow the instructions on the tool.


   Converting a 9.9 to 15 HP :  This question seems to pop up quite frequently and can get controversial from those who have never been associated with the conversion and are only going from what they have heard.   As you can see from the above information in this article, it is possible.  New carburetors are expensive if even available, (over $200), but used ones go for half that, then you can probably expect to rebuild it.  Some will tell you that you need different reed valves and reed valve stops.  I bought a 9.9 new in 1978 and still used it until I recently sold it.  It is the basis initially for writing this article, as I was trying to convert to 15 hp and there was no information, or reliable information available.  I also bought the repair and parts manuals at the same time.  From this parts manual I can assure you that the reed valves for either engine are the same for that year.   However in 1979 a .0125" shim was introduced that was placed between the leaf valve stop plate and the leaf valve itself for the 15hp motors only.

   I have not truly tested a before /after on the same boat, same water conditions.  But will speculate that it may not be worth the price or effort to change the exhaust tube when upgrading on the older motors, if trying to up the hp to the full blown 15hp unless you can find a used tube.  Remember that the older (pre 1981s) motors still used the same 9.9 exhaust housing on both motors.    I have compared the early 9.9 exhaust tube with the later and 15hp tube.  The mounting flange where it bolts to the powerhead is different, but it appears that the 15hp can be installed and would fit.   But just by changing the housing itself, you will not show many benefits.  Tuned exhausts are well known in racing two stroke motorcycles and are very effective at specific RPMs but need to be matched to the carburetor also.  


    Also if your boat is too large, heavy, or you and your fishing buddy plus gear are overloading the boat, your conversion benefits will not show.  However if you have a lighter boat and it just needs a nudge to get it onto a plane, then possibly going to the 15hp would help.

I will not go into exact part numbers for this conversion, as depending on your year of motor, a simple conversion could mean just a carb on say a 1977.  But if you wanted to wring every last bit out of it, you could also change the exhaust tube (3 types), reed valve stop shims and prop. Then if you had a 1990 9.9hp which is basically a detuned 15hp, just a carb may do it for you.


   The larger carburetor will gain the most in increased RPM, (about 1000 RPM more, according to OMC) which in this case also relates to more horsepower.  If you insist on squeezing that last ounce of energy, then maybe the aftermarket reed valves, polishing and deburring the intake manifold, replacing the exhaust housing will be of some benefit.  But, if you are trying to push it this far, at wide open throttle for an extended period of time, my thoughts are that then the rest of the engine may not stand up for long, so go to a larger motor.


    Props are listed the same for both the 9.9 and 15 hp motors, however depending on the weight of your boat, it may be advantageous to try a lesser pitch version.


   Also, as mentioned, the inner round exhaust tube was added after about 1981 for the 15 hp.  This exhaust tube on the 15hp has a circular cross-section and gets wider at the bottom, called a TUNED EXHAUST as opposed to the narrow, rectangular shape on the 9.9.  This was probably copied somewhat off the tuned 2 stroke racing motorcycle exhausts of that era.   Part of the principle was that the burned gasses also carried noise waves from the motor, if these waves created a backpressure where the actual exhaust gasses put noise pressure at the intake ports at the programmed RPM to block fuel leakage at the same time having a more efficient exhaust so it could escape more easily.  When a motor breathes better, this equals more horsepower.  This is a rather complicated system using sound waves.


   Then in about 1987 another exhaust tube was brought out, this one is a short rounded unit and is used for both the 9.9 and the 15 hp motors.  My reasoning for the 87 date is that the 86 parts list still shows the old housings, where the 1990 list has the newer one.  This motor underwent many changes in 1987 as to thermostat location, fuel pump, twist grip throttle, etc. where they actually produced a 15hp and detuned it to a 9.9hp, as compared to the other way around up to then.

   On the later motors from late 1987 on thru the 1992 versions, when they changed carburetors, the inner exhaust tube is quite a bit shorter than the early ones and this same part is identical for both the 9.9 and the 15 hp versions.  Why, I am not sure at this point in time, unless it simply saves money on parts inventory and they found they gained enough by the other minor changes.


   So, In Summary of Converting to 15 HP :  If I have overloaded you with information on the differences between the 9.9 and 15hp, sorry.   I guess it is just how far you want to go in changing parts to do a partial or a total conversion.  Any changes will probably make a difference, (whether measurable or not) but to do the full blown conversion, ALL will need to be utilized.  However depending on the year of motor you have and your needs, a full blown total conversion may not be cost effective.  I have not listed part nomenclature and number for each change that they did over the 18 years of production of this series because of the various changes and some could have been superseded by other changes.  If you are that interested you will have to go back and re-read what I have posted here, maybe many times.  And then finding the correct part may be another issue.  I personally would have to really think a lot about doing it to say a 1974 motor, whereas if it was a 1988 +, that may be a more worthwhile project.


    A reader recently made the conversion on a 1976 9.9hp Johnson using the carburetor off a 1980 15hp Evinrude.  "The carburetor change upgrade results were fantastic.   21mph GPS top speed with 9.9hp went to 27mph with the 15hp conversion on the same boat.  The reed valve shim and the 15hp exhaust will add a bit, but are not necessary to make a marked improvement to a old 15.    Do not get confused with people saying the intakes are different, or the head is different or the ignition is different.........THEY ARE NOT.   90% is all CARB".


   Another testimony taken from the i-Boats forum, "I had a 9.9 Johnson 2 stroke on a 16' Grumman V hull, it would start to get on plane but that's about it, it needed just a bit more power.  This was in a bare 4 bench seat hull with two 250 lb men aboard and a full 6 gallon tank.

   I eventually converted it to a 15hp by changing the carburetor and exhaust tube, that did the trick but it still only did about 18 mph

   Converting Over to a Full Electric Start/Charge System :  This can be done, and is covered under it's own article.  To access it CLICK HERE

   Shaft Length Differences :   The standard motor comes to mount on a regular boat's 15" transom.  This is designed so that with the motor mounted on the transom, the bottom of the boat is even with the cavitation plate of the motor.  The cavitation plate is the flat fin in the lower unit housing above the prop.  The difference in the longshaft unit is that it has a 5” aluminum extension the same physical size as the exhaust housing, to fit a 20" transom.  This unit is supplied as a kit, comprising the extension, a new 5” longer driveshaft, a 5” longer shifting linkage rod, a copper water tube long enough to couple between the water pump & the old tube from the power head and 6 more bolts to assemble it to the gear case.  

1978 Regular, or 15" Short shaft

1986,  20" Long shaft

1988 Sailmaster, 25" Extra Long shaft

Notice no extension Here is a 5' extension And here is a 10" extension


   The SailMaster is basically the same 9.9 motor with electric start but with a 10" shaft extension for use on sailboats.  My 1981 sales literature says the SailMaster has a heavy duty reverse gears with a higher reverse thrust.  The parts list does not substantiate this however.  I suspect they simply used a lower pitch prop.   The Yachtwin, was Evinrude's version of the Johnson Sailmaster, same motor just a different color and name on the cowling.


   Short Shaft Converted to Long Shaft :  This is a common question, and to try to condense some of these repair articles I have moved this section onto it's own "Shaft length Conversions" section.


   Repaint :  You can purchase OMC Johnson original white paint #777171 specified for 1981-1990 at about $11.50 a can.   Tempo #0204601, (Johnson White 1981 to Present) which is an aftermarket marine paint for about $1.00 less, or Krylon brand which is available in about any hardware or automotive store, in about any color you want, it may not match exactly the original color however, but it is less than 1/2 the price.  I am torn between the Krylon brand and the original, as I like the idea of being able to touch up paint scratches if the need arises.  Krylon's Ivory Gloss #1504, is very close to the standard white that most of  many of these later motors were painted with.  I have found with the OMC paint, do not try to do thick coverage or it MAY wrinkle.  Do many thin coats. 


   The only drawback using a non-original paint is that it may not be as resistant to gasoline and gooey black oil as the factory or replacement types.  A cure for this is then a clear overcoat of Duplicolor 500 degree clear Ceramic Engine Paint from NAPA  be used.


   The 1974 Johnson OD green color was changed to white in 1977I have not found a good Evinrude metallic blue aftermarket paint.


    If you decide to repaint the cowling and the rubber motor seal is OK, you can pry it away from the fiberglass or plastic depending on the year, placing matchsticks between the rubber and the fiberglass to allow for a better paint job to go under the seal.  This original rubber seal is stapled onto the fiberglass.  If you need to replace the seal (about $40.00) after repainting, to reinstall it instead of trying to staple it, just use a latex adhesive and many old wooden spring type clothes pins until the adhesive dries.

    If you plan on replacing the decals, you will probably not be able to find any factory available earlier than about 1993 or 1994.  Price is about $40 for these original replacements.  If you purchase these later model decals, you will have to cut them to fit, as the newer side decals sets lower than you might think.  This is because the motors have a upper cowling that the seam is more horizontal instead of at an angle on the pre 93's.  This changes the position of the decals if used on an older cowling.  You will have to trim some off the front bottom at an angle of these side decals to get the lettering horizontally positioned.  And you will also have to trim the bottom rear decal to be somewhat close to a decent position.  Lay them out and tape them on the cowling before you install them.   You will not get a decal for the front where the control knobs are on these newer decals as there are none used on these later models. 


    There has recently became available on e-Bay, aftermarket vinyl decals that are generic for this series of motors at a price of about $14.00 to $38.50 a set.  These generic ones are quite good, a close coloration to the later originals, but do not match any specific year.  They have the "Sea Horse"  and appear to be die or laser cut.  They are made for both the Johnson and Evinrude  9.9 and the 15 hp.  The picture below appears to be on a repainted 1983, as it does have twist handle gears and the side idle control knob.  These aftermarket ones only have decals for the back and sides, not the choke indicator, shift positions, trolling speed indicator, etc.


    The later motors from about 1987 had the power head, flywheel and prop painted black.


Aftermarket Decals on a 1983 motor 1983 motor, note the gray lower sections with the original idle & latch decals in white  


   Commercial Motors :  I am making possible exceptions here.  Anyone who can clarify, please contact me.  Initially I thought all the gray painted lower sections were commercial motors, now I am not sure.  I can verify that at least a 1983 9.9hp and a 1984 6hp recreational motors were painted those colors.  So maybe somewhere along the line certain years and sizes of recreational Johnson motors were painted like this.   You can tell if the gray is original if the instruction decals are in white (as seen above) instead of black lettering.   This touchup paint is labeled "platinum gray" and listed for motors from 1984 to 1987.


   There is mention in the Sierra and NAPA catalogs of "Commercial" 10 and 14 hp motors.  According to the NAPA catalog, they were made from about 1988 to 1991.  It appears these are cheaper units.  They essentially are the same motor as the 9.9 or 15hp, but will have only a rope starter and with a tiller throttle handle system only.  They used the older CD ignition system instead of the UFI as used on the regular motors at that same time.  There was no provision for electric start or a charging system.   


   Some even use the older non thru the prop exhaust lower unit very similar to the older QD series.  And the NAPA marine parts book verifies this by listing gearcase O-Rings for these as the same as the 1957-1963.   My guess is that these were a sort of a "Clean Up" run made of older parts and sold thru some major discount stores, or for export out of the states.


   Non North American Made Motors :  European Johnson/Evinrude motors did not follow the year-on-year model changes as their American counterparts.  So, if you are working on one of these, and have a Clymer or Seloc manual, read it with caution, but be prepared if a few things might be slightly different.  Australian motors can also be included in this group.  The suspicion here is that any left over motors at year end, got pushed to other countries.   Also there are usually additional letters in the model plate ID.  If it was a Canadian motor, there would have been a prefix letter "C" on the model number as in,  CJ10RCDB.    For more detailed information on worldwide production, CLICK HERE


   Different Model Designation :  Sometimes odd-ball motors pop out of the woodwork.  Here is an interesting bit of information, for a Johnson "GT10" which apparently is actually a "restricted" 15hp motor,  but not a true 9.9hp as we know it here in the states.   These are usually seen in Europe or Australia, however I have heard of one from Canada.  Ones I am familiar with were actually an actual 15hp motor but with a restrictor plate between the carburetor and the intake manifold apparently to economically change the power to 10hp.    These motors seem to be made using pre 93 OMC 9.9 or 15hp parts even if the motor was made later, kind of like a cleanup project into other parts of the world.  

   In doing internet an search, all I saw were the pre-93 motors, however having a model number with some dating the motors into 2006.  This being the case, I suspect that when the 93 models came out, the left over parts were shipped to Belgium and assembled there under the GT10 designation.  As in this search, I have seen the deep metallic gray color on these GT10s that is conducive to 2000+ motors.

   In my correspondence with these owners, the motors seem to have been on the market under only the Johnson brand at least in 1988 and up to 2006.  Now they also tell me that in some countries, normal operation of a boat with over a specific HP, you need to take an extensive test and be relieved of some weight in your wallet.  It seems that depending on the country this HP can be from 6 to 10hp.  So maybe the motor manufacturers are detuning existing motors to come in under that rating??

   The restrictor in the photo below, came from model number BJ10SPCEC, which places it as a 1989.  This one is from Europe, made in Belgium, short shaft, electric start and with an AC lighting outlet.   Which is odd because usually the electric start motors did not have the AC lighting provision.  

   I would think trying to troll with a motor modified in this manner may not be ideal even though the carburetor would be adjusted downward to any possible idle smoothness matching the air flow.  But without knowing the history behind these, we may never know.  So if any reader has any more information, please contact me so I can correct my suspicions or update this info.

Here is the restrictor plate mentioned above.

   I have also corresponded with a gentleman in Australia who has a 1979 15hp Evinrude that should have electronic ignition (if it was here in the states).  However it has what appears to be original magneto ignition, which is 3 years after that was changed over here in the states.  It also sports the 1974 head.  His ID plate reads, Evinrude Motors, Bankstown, New South Wales, Australia. A division of Outboard Marine Corporation Pty Ltd., Aus.  Again, a chance to use up surplus parts in other parts of the world.  Talk about a non OMC repair man's nightmares who does not have access to factory manuals in identifying parts, 20 or 30 years later.


   Real Oddballs :  You may also encounter motors that you can not identify.  These may consist of a 9.9 or 15hp powerhead mounted on a older 10/15/18hp complete exhaust housing lower unit.  Here the HP will not be consistent with US models, like a 9 or 14hp.  And the model number does not even show in the factory lists.  These appear to be a mix-match of left over parts that were assembled and or shipped to distant parts of the world.  And one seemingly common denominator is that all I have seen or heard of are all Evinrudes.  Some of these are common if made in Hong Kong, and model numbers do not coincide with any published information.  About the only way to somewhat match things up is to carefully observe the motor UNDER the cowling.  This would include rope starter type, flywheel, ignition, carburetor, whether plastic topped or not, thermostat placement, coils, upper cowling  shape indicating after 1986 to accommodate the top thermostat, and throttle twist grip/kill button.  These may all help date the powerhead.


   In the motor shown in the photos below, the name "Worktwin" is indicative of a name used prior to 1975.  The electronics are magneto style, the midsection appears to be from a 10hp QD or 15/18hp FD model made for a 5 bolt lower unit and the extension for the gearbox indicates it is a later 4 bolt gearbox.   The powerhead was made starting in 1975, with the magneto being replaced in 1977 by electronic ignition.  There appears to be some red plastic at the rope starter that does not seem normal for those years.  The powerhead's lower pan appears to be modified to allow the throttle linkage to align with the earlier FD twist grip throttle linkage.  The midsection is definitely off an earlier motor.  The upper cowling  is off a Evinrude later than 1975 or up to 1987. 


    But notice the excellent condition of the upper cowling, this motor just surfaced in 2014, so my guess is that this motor has been setting for a GOOD NUMBER of years.


    The model number is 90903R which does not come up anywhere.  My guess is that this marina in Australia acquired these misfits for a rental fleet.


Upper Cowling Here shows the mix-match of OMC parts

   You may even find some motors designed to run on Kerosene.  Apparently they were designed to be sold in underdeveloped countries where gasoline was hard to obtain, but where Kerosene was used for heating oil.  They usually have 2 headgaskets to lower the compression, used hotter spark plugs and possibly a small internal gas tank for starting, which was then switched over to kerosene after it is running.  These motors if shipped from the factories as a Kerosene motor would have had a "K" as a prefix to the model number.  An interesting thing here is that Kerosene has a LONG shelf life, is less of a fire hazard, so it is well accepted as propellant for sail boats that are at sea for extended periods of time. 

   AC Output :  Some of these motors (approximately 77-92?) could have been fitted with different stator (electric start) assembly under the flywheel that gave a AC output voltage.   We may not see any here in the US, but about all the European and Asian motors seem to be equipped with this AC voltage feature and is normally used for the running lights on the boat that the motor does not have 12 volt electric start.  The repair manuals say the regular output to the power pack is 300 volts AC,  for these motors, a converter is needed to make it charge a 12 volt battery.  Without me having a owners manual, I am assuming that the light bulbs would be 240 volt, which I understand is common in Europe.


    This appears to a be a long way around to accomplish an effect, but then maybe we don't understand their ways either.  These motors appear to have the electric start ignition under the flywheel and the junction block on the side of the motor for the wires, no rectifier converting the voltage to 12 volt, but for the rope start versions NO electric start flywheel .


    The parts manual indicates that these stators were for motors with serial number prefixes of A, B, C, or E if used on an electric start model, while the prefix would have been H if on a rope starter version.  This manual says "Serial Number prefix" however it would seem more in line with how things were done IF it was a prefix to the model number.  There was a plug in outlet on the LH rear side of the upper lower cowling that had a rubber plug cap the covered the outlet.  


    In the two photos on the left below, the cream/red coils are for the AC lighting.  In the center photo the ignition coil is the black one and the pickup timing module is the whitish unit on the far front side.


Top view of magneto ignition & AC coils Here is the newer CDI ignition & AC coils Decal on top of cowling


Plug in on port side

  Another view of the plug in on port side

   The above pictures were supplied by a gentleman from Germany who has an Evinrude E10BACNS motor that has AC lighting.  From this I conclude to mean  E = Evinrude  10 = HP  B = Belgium  A = AC Lighting, Rope Start   CN = 1982  S = Model revision


   Another motor, an Australian Evinrude, model  AE15BAESR was referenced in an e-mail inquiry from gentleman there that had the same 3 wire plug coming out the side.  His conclusion is as follows A = Australia  E = Evinrude  15 = HP  BA = AC Lighting, Rope Start   ES = 1990  R = Model revision.  


   Another model CJ10SPCEC on the ID plate, but the decal on the cowling says GT10.  From how I would read this would be the C, meaning Canadian J = Johnson, 10 would be a 9.9 powerhead (however it and the 15hp shared the same block) the SP = could equate to Special, the CE = 1989 and the last C would be the model revision.   And since there were a special run, would NOT have had an electric starter, but a rope starter.  


   Here is some info that I recently garnered off e-bay.   "OMC,  AC lighting kit #584864 kit,  fits 1993-2000 9.9 & 15 hp 2 stroke engines.  This kit provides the ability to power a boat's lighting system without a battery.   Current is AC only and cannot be connected to a battery".


   Ongoing Improvements :   Other than the before mentioned changes, there have been ongoing improvements in this model since it's inception.  The throttle cable twist grip system incorporated in about 1986, initially still used the original kill button.  In 1987 the kill button was moved to the end of the twist grip control. There is a problem with this type, if some makes of extended tiller handles are used that the owner then can not reach the kill button when these handles are installed.  In 1991 the kill button was moved again to the middle of the steering arm near the rear of the twist grip, it utilizes the safety lanyard disconnect "Man Over Board" kill switch.  If this safety lanyard/clip is missing, or pulled out, the motor will not start.  If you are using the motor for a auxiliary motor, the safety type unit may just get in the way, so you can then purchase a override, which is a small red plastic horseshoe clip called  "clip-restart" #431808 for about $3.60 which makes a neat configuration which allows the motor to run, while still allowing the kill button to be activated.


    The SailMaster motors used a metal 3/16" rod that came out of the front cowling above the rope starter handle, went across and back in on the other side of the handle.  It was slightly arched upward in the middle.  Apparently this was to prevent the starter rope from becoming readily frayed if the motor was over the stern and lower on the transom than on regular boats.


    Each year had a different decal, so if you have a good memory, you could identify each year this way.  The earlier Evinrudes had a different motor cowling, in that it was slightly radiused on the front, back and had a slight hump in the middle.  The top cowling for the Johnsons appears to be change slightly in about 1987, in that the upper corners have a bevel of a little over an inch all the way around the top, they got away from the squarish shape of before.


1975 Evinrude 1989 Johnson

   HP Rating Changes :   Another point.   Up through approximately the 1985, the engines were horsepower rated at the crankshaft.  Around 86, they had their horsepower rated at the propshaft.   This came about because of the imported outboards were outrunning the domestic motors of the same horsepower as the imports were rated differently.  This meant that the 86 and up had to make about 10% more real horsepower than the 85's due to the inherent horsepower loss in the lower unit gearbox.   My guess is that is why OMC made changes going to different carburetors at about mid year of 1986, trying to increase horsepower without major changes to the powerhead.  This gave them breathing room to do a complete engineering change for 1993 when they upped cubic inch displacement and horsepower along with a newly designed complete upper part of the motor.

   The piston diameter for both the 9.9 and 15 hp motors from 1974 to 1992 are 2.188" with a stroke length of 1.760" which equals 13.2 cubic inches or 216 cubic centimeters.  Full power operating RPM for the 9.9 is 4500 - 5500.  The 15 hp motor is rated at 5500 -7000 RPM according to a 1978 Johnson sales brochure.

   Sometimes referred to by marine mechanics as the "Big Block" 9.9s, the piston diameter for the post 1993s was increased to 2.365" requiring a different head gasket and the displacement was upped to 15.6 cubic inches or 255.64 cubic centimeters.  The .015" spacers under the reed valve stops designed for the 15 HP was then used for both motors in this era.   So now the basic motor has been redesigned to increase the HP for the 15hp unit without the add ons required to achieve it's rating, with the 9.9 now being the "detuned" carburetor version.


   Recommended Spare Parts :  If I was in a location where spare parts may be harder to come by, I would consider stocking the following spare parts and in the following order.  Spare spark plugs would always be a requirement.  (1) fuel pump filter cap and gasket  (2) starter rope  (3) flywheel key  (4) small fuel hose clamps  (5) lower unit drain plug seals  (6) spare prop, nut and cotter pin  (7) water pump impeller  (8) fuel line primer bulb.  If for the older 1974 to 1976, then I would add points and condensers.


   Replacement Parts from Bombardier :  OMC sold out to the Canadian company Bombardier Corp.  in Feb.  2001.  Replacement parts for some of the older motors has dried up somewhat as the new owners may not be as committed to supplying older parts as under the previous ownership.   Now when ordering replacement parts, the dealer will see a new listing in place of  the older N L A This new listing is Vintage, which actually means No Longer Available.  Bombardier also changed the discount structure for many parts.  The retail prices are still listed, but the dealers get less of a discount and they now have to pay the shipping no matter what status they have.   Their website is very non-user friendly if you are trying to find parts.  The links do back to the preceding page, but if you do manage to get to a parts list page, there is no way to identify the hp,  just the year of the motor, so you have to guess.


   Replacement Parts from Internet Bombardier Supplier : There is one marine dealer that is set up to do internet sales and they have a very complete website that is user friendly with the availability to call up exploded views of the different parts for even motors 30 years old.   They have illustration numbers and corresponding actual OMC part numbers and prices.  Even list if any are no longer available.   Here is the link to their website


   Another one I have found that is fast and the price is VERY competitive is   They carry only aftermarket parts, like coils, rings, gaskets, powerpacks, carb and fuel pump kits etc.   They do not handle throttle linkage, clamp screws, decals or many of the individual needed parts.  They are cheaper than the NAPA or Sierra but use the same part numbers and you can specify which shipping you require.   The last electrical items I priced from a marine dealer for original parts was $160.  Sierra thru an automotive store was $120 with my friendly discount, plus freight, and iboats was $106 including freight.


   Replacement Aftermarket Parts : The Sierra catalog is available from Car Quest stores and  NAPA the catalog is available from them of course.  Many times you can get a part a lot faster by going thru these automotive stores, in that they are probably moving a lot more parts than a marine mechanic and your order can get called in almost daily.  But be advised that they will have to add on shipping, normally do UPS or FEDX air to keep their mechanics happy.  So, unless you specify that you do not need it ASAP, your shipping will be more expensive.


   One thing I have also found, is that these automotive parts dealers are just that, they usually are not knowledgeable at all in the outboard applications that you may require.  If you ordered off their catalog and ordered wrong, it is your responsibility to pay for it even if it is the wrong item.  I went this route once, ordered a thermostat kit #18-3674, thinking that I also needed other parts than just the thermostat that I might not have been aware of.  What I got was the thermostat along every gasket, spring, seal etc. that would allow me to fit it to MANY different models, for a price of $25.26.  I found out later that the thermostat alone #18-3672 (all that I really needed) would have been $11.86. 


   Also some of the NAPA gasket sets are not complete or they are misleading as to years covered.  They list the gasket set as fitting 1974-1992, WRONGFor motors prior to 1977 if you order a gasket set, the water cover gasket, (the one the thermostat is under) gasket, is not supplied if you order the set, as there are 2 different water cover gaskets as described previously.  The older one from 1974 to about 1977, OMC #318917, is (NAPA #18-0397).  The later one from 1978 up to 1986, OMC #321387 which is they (NAPA) shows as #18-2905, and which they call "water cover gasket" is the one that comes in the overhaul gasket set, not both as you would think if there was an overlap.


   In reality I have found #18-0397  that they call "thermostat gasket" is the proper one, which OMC calls "water cover gasket".  Also If you order #18-0112 that NAPA calls the "powerhead mounting gasket",  I have not been able to find where it goes on this motor, but in reality I  really wanted #18-2907 that they call an "adapter housing gasket" that OMC calls "powerhead to exhaust housing gasket".   Maybe this  #18-0112 might just be one that fits some of the odd "parts" motors using a pre 74 exhaust housing and lower unit.  So buyer beware!!!

   One comment here on the electronic transistorized conversion kit.   I was at one time was quoted at about $60 for this unit.   Also from the experience of a marine dealer that says it does not work well if it is being used for trolling, as it appears the RPM is too low to give adequate ignition.


   Also be prepared to pay the shipping and handling costs, with an average about $6.00 for a $23.42 gasket set.  Where if you ordered it from a marine dealer and it was shipped with his other parts orders, he may well equal out the shipping costs.

isted below are some of the normally used parts found in the respective Marine Catalogs.  It is found that they both use the same part numbers.  The bold numbers are current prices as of 7-2004.

Electronic conversion kit 1974 1976 #18-1501    $60.00    Head gasket 1974-1992 #18-2963   $  7.80
Power pack 1977-1984  #18-5758   $72.08    Gasket set 1974-1992  ** #18-4306   $23.42
Coil  1974-1976 #18-5196   $33.86    Rewind spring 1974-1978  #18-6503   $28.46
Coil  1977-1984 #18-5176   $25.48    Rewind spring 1979-1992 #18-6522   $24.35
Coil  1985-1992 #18-5179   $22.67    Prop nut #18-3706   $  2.57
Coil  1993 up #18-5170   $34.17    Cotter pin #18-3740   $    .44
Condenser 1974-1976  #18-5205    $  3.29    Universal manual starter rope & handle #18-4904   $  6.92
Tune Kit 1974-1976 #18-5003   $29.25    Piston, Standard, 1981-1993, with       rings, wrist  pins & circlips #18-4106   $68.81
Carburetor kit 1974-1987 #18-7223   $12.75    Rings,  Standard 2.188 dia. bore #18-3931 .
Carburetor kit 1988-1996 #18-7219    $21.86    Rod bearing cage with needles 74-83   #18-1359   $25.96
Fuel pump kit 1982-1986 #18-7823   $11.86    Center main bearing 1974-1983  #18-1359   $25.96
Water pump impeller 1974 up #18-3050   $10.00    Electric starter  1974-1992 #18-5617 $225.26
Thermostat 1974-1986  #18-3553   $11.86    Crankshaft  seal kit #18-4332   $17.06
Water cover gasket 74-76 #18-0397   $ 1.99    Lower Unit drain/fill plug screw #18-2387   $3  .16
Water cover gasket 78-85 #18-2905   $ 1.00    Lower unit plug seal (washer) .   $    .50
Bypass cover gasket- all #18-0967   $ 2.37 ** wrong, - should be only from 78-92, as it does not have right water cover gasket #18-0397 . .


    Replacement Parts :  The one online marine dealer that seems to have factory parts for most models is .  Their website has a online illustrated parts listing.  Their parts are discounted off retail prices.  They even have a parts technician that you can call to verify if you encounter any questions.   I am not sure how old of models that they carry parts for, but I had no problem getting water pump parts for a 1977 Mercury 4.5 hp.  They then ship FedEx.


   The Internet and e-Bay can also be a source of new or used parts.  However, if you are on e-Bay, then you had better be darned sure that you are bidding on the right part, as many of these sellers do not know the fine details as to interchangeability and they must think that just because it is a 9.9 that everything else must also fit.  Or they read somewhere that all the water-pumps are the same, so assume that everything else is.   I have tried on occasion to inform some, only to be told I was wrong and to keep my nose out of their business.   As you have more than likely concluded after reading this article that it is NOT ONE SIZE FITS ALL, especially in the electronics.  


    A few years ago, I bought engine parts off e-Bay that were advertised as fitting a  9.9, but the picture quality was poor, then after I received them, NOT  9.9 parts, so after much research and comparison, it appears they may have been for a 1989  6 hp instead.  The seller probably got them in a trade and had no real idea what they really fit.   Close only counts in horse shoes and hand grenades.


  I am still learning about some things pertaining to these motors even after 40 years+, so information may change often.   And if I happen to answer a on-line question from a reader that I may have not thoroughly covered, many times I rewrite or add info & photos clarifying the issue.   I do wish to thank some of the e-Bay sellers from whom I borrowed some of the above motor pictures.

Copyright © 1998 - 2020 LeeRoy Wisner  All Rights Reserved

Back to the Main Ramblings Page


Originally stated 06-1998, Last Updated   11-05-2020
Contact the author

If you try t