Outboard Jet Maintenance





    For those of you who run a outboard jet on your boat, there is some required maintenance to keep it functioning properly, and to assure that you can remove, adjust or replace needed parts if required.    I am are not covering inboard jets in this article.   I will not really go into the a lot of details here of history, how it works or installation as it is available on the manufacturers website listed below.


   The bulk of all the current outboard jets are made by Specialty Mfg Co..  The jet units you find on the factory fitted motors, no matter what name is on the motor is made by this company.    Below is a working view of the unit (taken off  this company's website).    There is at least one other newcomer, Durajet, that is using a design very similar to a airplane jet engine in the location of the prop, sucking the water in the front & out the rear.

   This Specialty Mfg company was started by Dick Stallman after he experimented with an idea in 1960.   Dick is still with the company as of 2008 as their engineer.  The refined product you see today is the result of his efforts.   Dick was good friends with the Wooldridge family boat company from the inception and both being far sighted, they did a lot to get us where we are today.  Stallman's oldest jet motor is proudly displayed in the showroom of Wooldridge Boats in Seattle, Washington. today.


   The different series intakes use replaceable liners of  6 5/8", 6 7/8" and 7 3/8".


    A brief description of the units for OMC, the series would usually be "A" for 40 to 60hp 2 cyl, "L "series 55 to 70hp 3 cyl, "R" series 85 to 250hp V4 & V6.

For Mercury the series would be "E" for 40 to 50hp 4 cyl, "A" series for 3 cyl for 40 to 115hp & "S" series for 135  to 220 hp V6.


   They build over 80 different models that fit 6 different outboard motor manufacturers.


   On about all of these units the old prop water pumps are transferred from the prop lower unit and are used on these jet units.  So if you would need a new water pump impeller, it will be the same as your motor was before it had the jet unit installed.


    Most of these units will be installed on outboard motors which have steering wheel and console controls.  However you will see some tiller mounted motors with jets attached, especially in the Pacific Northwest and being used mainly by river guides.  They function the same, with a few differences as defined below.


    The exploded view of the design shown below was taken from Specialty Mfg Co. website.



Brief Operation Course :  These paragraphs are for those who have not read the main jet article.  These motors do not really have a neutral because the drive impeller is connected directly to the driveshaft.  There is no gearbox.  The reverse and FALSE neutral is accomplished by operating the rear clamshell which diverts the water either straight out to the rear for forward when the clamshell is down, or when up for reverse.  Trying to find a neutral may well be tricky because you are trying to split and equalize both the forward and reverse thrusts by location of this clamshell.  Therefore if you start it up at the boat launch, most 2 cycle motors need a warm-up period before they will allow you to put them in forward without them dying.  You will see some operators leave the boat on the trailer, start the motor allowing the motor to push it against the trailer's bow support.  Or they may launch and start away from the launch, but nose the boat into the bank with the motor running to allow it to warm up.


    Since these boats need to be a fairly shallow bottom, the steering if in a sharp corner can be a problem.  At a high speed since there is no prop or lower unit hanging deeper in the water, the boat will slide on a corner if going fairly fast.  It also is possible for the boat's inside corner chine to dig into the water allowing the boat to flip over.  Partly because of no or a minimal keel.  Some boats have ribs on the outside bottom to allow the boat to "dig" in a little better.


    Do not try to shift from full forward to full reverse as there are no gears, just the clamshell, it will not hurt the motor.  BUT there may not be anyone left in the boat afterwards.  Kind of like hitting as brick wall, everything not tied down will be ejected forward.


     Reloading at a launch where there may be a wind blowing can also be a problem as since you have no neutral you can not slide up to the trailer, shift in to neutral, let it set for a few seconds, then re-power to move onto the trailer.  You will see many sled type trailers with side bunks which help guide the boat centered onto the trailer.  Here you would normally use the current or wind to help you by nosing in to the wind and if your trailer has side guides, allow the bow to bump the opposite side from where you are entering on.  Once the bow is in contact with this guide, turn the motor so that the stern comes around allowing the bow to now be guided onto the center of the trailer.


On The Water Repairs :   The one thing I will say is that it is very beneficial to have a motor that has power tilt.   Jets don't like weeds or sticks.   Depending on the quantity of floating weeds, running thru them at speed may work of you have a Vee bottom, but outboard jets are usually on flat or a slight Vee boat and more often than not, you WILL clog the intake grill if there are many of them.  The intake will plug up, the motor will rev up but the boat will not go anywhere because you have reduced the intake water, or if it does it will be a a drastically reduced speed.


    Many times if you just shut off the engine and let it set for a minute, start it back up, see if you regain power.   If the stop and go did not clear the obstruction, shut it down again, tilt it up, and reach back and clear the intake.  Be sure you are wearing you PDF.   It is usually not that hard to clean out the intake grill on an outboard jet.  You may want to keep a long handled screwdriver or something similar with a lanyard tied to it in case you drop it while you are hanging over the stern.


    Ingestion of rocks is another matter.  The smaller ones will go thru the impeller and out the rear with no problem other than possibly chewing on the impeller's leading edge.  Larger rocks however approaching a quarter size coin may well get wedged between the grill bars.  If that happens, a special pry bar with a cross pin needs to be fabricated so that you have leverage on the grill bars against the wedged rock.    Remember you are on the water, hanging over the stern trying to dislodge these rocks, so you might consider dropping anchor or get to a sheltered location.


Here is my highly scientific engineering designed grate cleaning tool


   If you are running in shallow mud infested water, watch the motor's overboard tell-tail indicator, (pee hole).  This water indicator HAS to be functioning otherwise you WILL get the motor hot enough due to no water cooling it and could burn the motor up, usually by seizing the pistons in the block.  If it just stops peeing and you caught it before the engine faltered, you might try shutting it down, wait a few minutes, then try again.  This may take 6-8 tries, but it usually works.  If not then you need to row, as the motor CAN NOT be run without water circulating thru the water passages.  You might try using a wire poked up inside the pee hole to see if just that indicator is plugged as water may be in the engine but not coming out a plugged pee hole.  Start it up again and watch this pee hole for water being squirted out in a steady stream.   If so then you are on your way.


No Forward Problem :  I had one reader who was using a remote console system, who had a problem with no forward, but only reverse.  He traced it down to a hidden rusty shifting cable that the outer sheath had exploded.   This was a recent purchase for him and apparently the boat had sat for a while, during which time the inner cable had rusted to the outer cable.  When he took it for his maiden voyage he was surprised in that he only had reverse because when he tried to shift it the outer cable exploded as shown in the photo below.  This could very well have been rusted in the opposite position.




  Off The Water Maintenance/Repairs :  To flush the motor, there is a 3/8" stainless bolt head on the LH side just below the juncture of the motor's midsection and where it bolts to the jet lower unit.  This is a flushing fitting hole.  Mercury sells a flushing adapter that uses a 3/8" National Course bolt thread on one end and a female garden hose end on the other.


    Mercury's Quick-Silver part number is  #24789A 1 with the actual name being "Flushing Device".  Also OMC Johnson/Evinrude uses the same adapter to back-flush for many of their outboard motors made from 1993 to 2007.   These adapters also fit about all of the Japanese outboards that use a back-flush system.


   You can remove this 3/8" plug bolt, screw the garden hose into the adapter, then into the motor, then turn on the water.  It does not need to be full force of the standard house water pressure, but about 1/2 force.   Start the motor up and let it run for long enough to get the motor warm enough to open the thermostat allowing water to flush thru the power-head.   Shut off the motor, then the water, replace the plug bolt.


Mercury Outboard Flush Device  #C-24789A 1


   Read your manual.  EVERY TIME you run the boat and come back in for the day, freshwater AND especially in saltwater, use the recommended low pressure grease gun and lube the impeller shaft's bearings that are located under the water pump. 


   This is accomplished by removing the rubber hose/grease nozzle that is snapped onto the Zerk fitting close by.   Some of the early units do not have this rubber tube but do have a 1/4" stainless round headed relief bolt that needs to be removed.  Do not use a regular hand pump grease gun , but the tube type gun that has the Lubriplate tube attached for less pressure.   Pump a couple of pumps, or until the Lubriplate grease starts to come out of the nozzle or hole.  Stop, wipe off the excess and replace the nozzle.   This nozzle is merely a cap covering the Zerk fitting.  On the opposite side of the unit as the grease fitting is a small hole in the base that you may never see, that is also a relief hole for the excess grease to be pushed out.   It is imperative that you grease the unit each time it is used to be sure that IF any water has gotten inside, that the new grease will force the water out and not allow this water to stay trapped inside giving the chance of rust to start in the bearings when the boat & motor may set idle for a while.


   Now, for those newbies to outboard jet operation, I recently had a question posed where the to be new owner to be who had problems identifying his situation.  He was concerned as to whether the "under the cowling hose water hose connection may not get water directly to the water pump and may result in the water pump being run dry"?  He did not trust his dealer if running the motor using this flush plug.  Finally it dawned on me what he was referring to, the block flush plug as compared to the jet unit flush plug.  The FLUSH plug on the side of the motor, are common now, but came out a lot later on outboards than the jet units which had their own flush system.  The block FLUSH plug bypasses the waterpump putting water into the motor water jackets, while the jet hose fitting supplies water to the water pump which is mounted on top of the jet unit.  However most block mounted FLUSH plugs are only designed to flush the motor when it is not running.  They do mention minimal running using this block flush, and not for extended periods of time.  There MAY be enough water doing it this way, being back-flushed to lubricate the water pump impeller ??  Also it depends on the brand of motor you are using.  If in doubt, consult your owners manual, but remember that this manual may have been written for prop motors.  Here for a jet unit, I would not use the block mounted flush plug if when RUNNING the motor, to ensure that the water pump impeller does not get damaged.


   Occasionally you may want to look up inside the grill grates to inspect the leading edges of the impeller blades.   If they have been chewed up enough by sand or rocks, there will be a disturbance in the water flow, if so, it is beneficial to remove the impeller and dress, (file) the leading edge of the blades.  While you are at it check the clearance of the impeller to the liner.   The factory suggested clearance is 1/32".   If you happen to run, or even start up in shallow water, the impeller will suck sand and small rocks up inside.   These act like grit and wear the soft aluminum impeller and liner.  The design is so that the impeller and liner are both made tapered, allowing the owner to adjust for wear, by shimming the impeller down, tightening the fit to the stationary liner.  When there is enough wear on either or both, the clearance increases, which decreases the efficiency of the unit. 


Here is a slightly dinged impeller front edge Here is a dressed up impeller blade front edge


   To repair this wear, pull the 5 bolts or 6 nuts (depending on the model) off holding the foot on, dropping the foot.   Remove the cotter pin in the impeller nut (again depending on the model) or end the impeller nut's lock washer flanges back allowing the nut to be loosened.   When you pull the nut and lock washer off, catch the flat washers that are under the nut.  Count them, REMEMBER THE NUMBER and place them separately in a secure place.   Try to pull the impeller off the shaft.   If it will not move, tap the impeller with a mallet in the opposite direction that it is normally rotating.  What has happened is the impeller's key may have became wedged against the flat on the driveshaft.  Tapping it backwards may loosen things enough to allow you to pull the impeller.


   If that does not do it, you might have to resort to a Oxy-Acetylene torch to heat the impeller.  In doing so you will ruin the nylon busing that is between the impeller and the drive shaft.  But at this point, it is sacrificial and inexpensive.  When purchasing a new bushing, you will need to know what size your jet unit is. On the RH side there will be (on the newer models anyway) a small aluminum plate held on by 2 nuts.   Here will be the model in alphabetical letters and the serial number.  The letter, or even 2  is what you need, like A, F, L or S etc.   If your unit is an older on without the model number affixed, then you will need to measure the shaft size at the impeller.


    If the above technique does not work to remove the impeller, you may have to use a small 3 jaw gear puller.  Place one jaw claw on each impeller blade, tighten the center screw against the drive shaft's center hole, then tighten until you get it to come up tight.   If by tightening a bit more and the impeller does not move, heat it again and try to pull using the puller while it is warm.   The white nylon bushing will begin to bubble and ooze out, but that is an indication you are softening it enough to possibly allow the pulling process to proceed.


    OK, you now have the impeller off.   There are more shims above the impeller, again COUNT THEM.  There should be a total of 9 or 10 shims (depending on the size of the unit) top AND bottom.   These units are designed with a tapered impeller and liner so that when you get wear between the impeller and the liner, that you can remove a shim below the impeller (and under the nut), move it above the impeller, pushing the impeller down tightening up the wear between it and the liner.


     Inspect the impeller, file the leading edge if needed according to the manual.  If there is wear and the clearance has increased you will want to shim the impeller down DECREASING the clearance to restore performance.  In reinstalling the impeller under this circumstance, you will need to take one of the washers from below the impeller, place it above the impeller.   This pushes the impeller deeper into the tapered liner, decreasing the possible excess clearance.  The clearance needed per side is approximately 1/32 of an inch (.032).  There may be some off center between impeller and liner due to manufacturing tolerances so check at the closest point.   Less than 1/32 of an inch is OK as long as the impeller does not rub.  More clearance than 1/32 of an inch will drop pressure and LOWER the jet's performance.


   You will not know just how many shims to move until you reattach the foot which contains the liner.  Move one shim, bolt the foot back in place with about 1/2 of the nuts, inspect the clearance.  If it is still loose, pull the foot and impeller off and move another shim from the bottom to the top.  Reattach the foot and inspect again.  If close, rotate the motor's flywheel to see if it is not bound up in any way.  If everything looks OK, remove the foot, bend the lock washer's tabs into the impeller's nut flats.   Reassemble the foot and you are back in business.


    IF there are major chunks of metal missing, have hope as you do not have to buy a new impeller.  Take it to a welder who is accomplished in Helicarc welding of aluminum.  He can build up the missing pieces so you can then file it back to near the original configuration.   Be careful to maintain the original shape and leading edge.


    Now one bit of advise.  On any of these bolts used to secure ANYTHING back together, clean the bolt threads with a wire wheel AND coat them with an anti-seize compound before you reassemble the unit.  You may have to take it apart later yourself, if not someone else will surely thank you.


Yearly Repairs :   It is recommended that every couple of years that you remove the unit from the motor, inspect, adjust the impeller if needed and if nothing else remove the bolts and reassemble them to assure that when you do need to disassemble this unit at the boat launch if the impeller needs shimmed or you get sticks imbedded in the impeller that you can get it off when needed without having to go to a marine repair shop for help. 


    This  yearly maintenance is even more important if you happen to fish in salt water or even bays, that depending on the tide and where you fish in the system, if there is some salinity in the water that you should do this repair every year.  The combination of stainless steel bolts screwed into aluminum is not very conducive to easy removal unless it was assembled using some form of anti-seize on the threads.   It may even be beneficial now to repaint the lower unit.   The older units were painted red, newer ones are usually black, while the ones supplied especially to the manufacturers will be the same color as the motors they are attached to.


   The foot has slots in it to accommodate the grates.   These grates, shown at the bottom of this article, are held in place by a 1/4" stainless steel rod going thru a hole on each end.  The front hole in the grate is a round hole, while the rear is elongated.  If this rod is not removed periodically, it too can become seized in the aluminum housing.  To remove a stubborn rod, you need a short 1/4" punch with a shank about 1 1/2" long.  This short punch will take a lot of pounding to the rod to start moving.  Once it starts, you can then use a 12" piece of 1/4" drill rod to drive it out the rest of the way.   When reassembly slightly grease this rod with boat trailer wheel bearing grease.


    When assembled ,the grates are flush with the foot's front but protrude about 3/8" below the foot's rear body.  This is probably designed to allow a stick or other debris to be carried down and away from the foot when the boat passes over it.


    Remove all the bolts, wire brush off the debris, and reassemble using some kind of anti seize on the threads.  You do not need a lot, but just enough to give some protection.


    One thread (depending on the model) that does not need a anti seize is the 6 nuts that hold the foot to the housing as they are stainless nuts attached to stainless studs.   These nuts should be stainless steel with the aircraft type nylon lock imbedded in the nut.  If not, replace them with this type.   It is a good idea to check these nuts occasionally as I have had one marine mechanic tell me that if the foot comes loose during the motor running, that bad and expensive things do happen. 


   When it is apart, do not worry if the drive shaft wobbles a little.  When it is installed in the motor, the upper end is aligned into the crankshaft's spines and things tighten up as designed.

    If you have to replace the bearing or the driveshaft, this can get interesting.  There is a large inside snap-ring that retains the upper seal unit inside the bearing housing.  This has to come off allowing everything to come out from the top.  An arbor press or a large lathe may need to be used top press the shaft and bearing out of the bearing housing.  Once you get the shaft and the bearing out, place each of the parts in sequence as they come apart.  There should be the snap ring of course, then the seal retainer, a aluminum ring spacer, a dished washer that has the dome up. 


    Now there is a outside snap ring holding the bearing onto the shaft.  That has to come off allowing the bearing to be pressed off the shaft.  Above the bearing is a spacer about 5/8" long on the shaft that has a machined inner groove on the top side.  This groove accommodates a .080 dia wire ring on the shaft.  This is the bearing stop.  The spacer needs to be re-installed with the groove up so the ring is hidden when the spacer is installed properly.  If you get it on backwards the snap ring will not have a grove to snap into on the shaft.


    In reassembly, the bearing has thrust one way so the writing on the bearing ring needs to be down, the domed washer needs to have the dome up, the the large spacer, the "L" series is .270 thick.  Then install the seal retainer and the snap ring.  It may be beneficial to slightly heat the bearing housing to allow the bearing to slide in easily.  The large snap ring has the upper edges slightly tapered.  It has to be in tight holding all the parts in tightly otherwise the shaft will ride up and down, (not a good thing with the tapered impeller).  You may have a time getting everything tight and at the same time the snap ring to lock into place.


    When reassembling the driveshaft and bearing housing into the main body, on the "L" series there are 4 bolts holding it in, but the bearing housing needs to be installed to align the housing grease holes with the grease holes in the main body.  There will be 3 small 1/8" holes that in the bearing housing have a slight counter bore around them that has a Neoprene O-Ring around the hole to act as as a seal.   These O-Rings are 5/16" inner hole diameter and 1/16" thick.   In the main body housing are mating holes.  Be sure that you have these holes align otherwise the grease will not be able to get into the bearings.  I have found that a small amount of non-hardening gasket cement on each O-Ring holds them in place during assembly.


Shown here is a "L" series jet unit, showing the shifter bracket & lever cam in position, the flush plug in place & grease tube, after a rebuild & repaint


    These "A" series have (5) 1/4" stainless steel Allen head bolts holding the foot onto the pump body, and the unit including the foot can be assembled completely, then just slid up into the motor and secured by the mounting bolts.  The "A" series also have a 3/4" shaft that the nut is retained by a cotter pin.


   The "L" series has (6) 5/16" stainless aircraft nuts attached to studs, but the bearing housing is bolted by 4 bolts installed thru the bottom opening onto the pump housing before the impeller is installed.  This means you can not install the impeller onto the shaft until you bolt the bearing housing to the pump housing.    The "L" series uses a 7/8" shaft and fold over tab lock washer to retain the nut.

  Other Adjustments :   The shift linkage is critical in that the cable's end needs to be set so that when the motor is in full forward  that the clamshell be clamped down tight against the bottom of the unit's exhaust tube.  If not, in forward movement thru the water, the clamshell will be pushed rearward and you will not get maximum speed as you will think you are in full forward as indicated by the throttle, but the clamshell will be pushed back enough as to have you in say 80 % forward but at the same time 20% reverse.  Not good.  (Take my word for this as I being a newbie a few years ago to jets, been there-done that).


   This above situation is usually seen on the old style that did not have the side locking cam plate.  Also since this impeller is always spinning when the motor is running, it is hard to find a neutral position so the boat does not want to move at least some.  Do not think that you can adjust the neutral position on the shifting control box  to match the motor's idle.   IMPOSSIBLE because of the previous paragraph.


   One thing that can be done as an improvement however, is that on most throttle/shift controls that are a single handle unit, the motor can not be shifted into forward at anything other than idle speed.  There is usually a lockout pin in these control boxes.  Removal of it allows you to (especially with a cold motor) to shift into forward to get away from shore at a slightly faster throttle speed thereby eliminating the usual scenario of the motor dying until it warms up.


Improvements :   There are a few things that can be added to improve performance.  By this if your boat/motor combo is not fitted exactly, your motor may cavitate (sucking air into the jet, creating over-revving of the motor).  This can be dangerous to the motor ALONG with if your impeller is not tightened down sufficiently, your impeller key may become worn, which over time, may shear completely off, leaving you DEAD IN THE WATER.


    Shown below are two simple but beneficial add-ons.   The intake side skirts and a aluminum bottom extension.  The 3/16" or 1/4" aluminum skirts are very beneficial if the boat's bottom is rather flat and the water has just enough small chop to allow the motor's intake to intermittently be airborne.  This allows repeated cavitation which means you either stay with it which allows the motor to over-rev, or slow down and pound thru the chop.   The side skirts (sometimes called Hot Shoes) provide just enough of a funnel to keep air from being sucked in on the sides.  They are installed in the same holes that the grate retainer pins are in.   The 1/4" stainless steel grate pins need to be shortened enough so that the original foot pin holes are tapped to 5/16" course threads and the bolts can be also used to secure the skirts and keep the pins from moving out.


    The bottom extension is made of 1/8" aluminum which provides an extension of the bottom that just lays on top of the front of the intake foot with some tension.  This is attached to the bottom of the transom which eliminates the chance of water coming rearward under full power, then being forced on top of the foot's front, which can then be forced back against the front of the foot and forced upward possibly spraying into the boat at high speed.


    You will notice in the RH photo below, the zinc anode bolted to the rear of the foot.   Clean this with a wire wheel or sandblaster to renew the effectiveness of this corrosion inhibitor, but do not paint it.


Outboard Jet "L" series on a 70 hp Johnson & an old 16' Hewescraft sled showing the bottom extension & side skirts Bottom rear view of anode, grates, impeller & side skirts


   Another improvement is to take the stainless steel grates and grind a chisel edge on one side.  This chisel edge is then installed down giving less resistance as the grates pass thru the water.   This edge does not need to be a sharp knife edge but merely a simple chisel shape.   Some boaters even do the same on the top side thinking they are eliminating a turbulence, hence a better flow.

   The newer grates have a slight bend on the front end.  This is so that when they are assembled in the foot, that there is some side tension on them, apparently to eliminating a possible rattle.  One end's hole is slightly elongated.  When replacing them use the round hole on the forward end of the foot first.  The elongated hole simply allows for a slightly less binding on reassembly or later on disassembly.


   You can also look at the aluminum impeller and if there are any roughness left from the sand casting process, using a air die grinder, remove them, again creating less resistance.  Also look inside the pump housing, remove any imperfections or protrusions at the same time, as I have found casting mismatches that needed to be ground out.  Remember these units are semi mass produced and anything you can do to improve a unrestricted water flow will improve your end performance.


Two modified grates on top & an unmodified on the bottom Cleaned up impeller casting imperfections


    I should not have to remind you that the motor has to be maintained in top condition otherwise all you have done to improve things on the bottom end will not really help.  Then there is the bottom of the hull that needs to be maintained also. 


Tiller Operated Motors :  The main difference here will be the motor is more free to turn right or left.  On motors designed for tiller operation, there is usually a set screw on the lower part of the pivot section that puts tension on the pivot shaft so the motor does not move as freely.  If your motor has been converted, it may not have this feature and probably not cost effective to retrofit it at this time.  One guide tells me he has no trouble EXCEPT when he is traveling towing the boat and motor down the road.  The motor wants to move all over the place.  So he just bungee cords it to one side.


   Some of these jet unit setups have a arc friction mounted adjustable tensioner on the motor to provide more control for this.  It may not be critical for the experienced operator, but for a newbie, and using a large motor, a small amount of overcorrection at the wrong time could be disasterous.

   Also since your steering is now a lot more sensitive, the rotation of this large centrifugal pump may tend to pull more to one side when running on flat water.   There are a thin fin inside the water exhaust outlet, top and bottom of the center engine exhaust tube.  You can bend the fins slightly at the very rear which will slightly deflect the water & can cut down on the pull to one side.


   These tiller units also need a special short shifting cable to operate the reverse clamshell.  I have been told that you can not shorten a regular shifting cable.  Well this old farm / logging / maintenance mechanic has done it.


Stainless Steel 4 Blade Impellers :  Wooldridge boats has come up with a improved thinner stainless steel 4 blade impeller that increases efficiency.  HOWEVER this is usually not a viable situation for motors below 120 hp when installed on heavier boats as the impeller is now so efficient that these lower powered motors now are underpowered and you may even loose performance over the original impeller.


Non On The Water Test Running These Units :  The OLDER units were not made with a flush adapter test unit. When I purchased my first outboard jet, I was a virgin, and totally uninformed of how these things operated, so when the seller slid a wheelbarrow up under the unit, filled it with water and started the motor, I thought this was standard procedure.  he only ran it for less than a minute because he could not keep enough water pouring into the wheelbarrow because of all the LARGE flow being blown out the exhaust and out of the wheelbarrow.  


    Boy did I get enlightened, that was not the prescribed method.  And not many old wooden cattle watering troughs still exist today.


   A cousin of mine had this old Mercury jet on his sled and he kept complaining that every time he wanted to start it up before season, he had to make arrangements with a neighbor friend to use his river gravel bar to back the trailer/boat down into the river to be able to start it up for the season.  I did a lot of information gathering, measuring distances on friends jet units and figured that if the factory devised a new method, that I could do a modification to these also.  Therefore read the following, where I did tear-downs and a lot of head scratching.


Modifications to Older Non-Flushable Units :  Many of these older units did not have the flush adapter plug hole and in order to run the motor, you had to back the trailer into a lake or river with the motor in the water enough to allow the water to be pumped up into the motor for cooling.  This can be a PAIN IN THE KATOOSH, especially if you are those of us who like to run the motor the night before we head off at 0-Dark-30 the next morning.  There is nothing I hate worse than getting to the launch, something does not work right while impatient boaters are giving you that unfriendly stare.


Flushing Conversions :  Reading the manufacturer's information on their website says these units can not be converted.   These units like many products on the market have undergone improvements as time went on and it appears this was one illustration.  The big problem is illustrated in the photos below is that there is one main cavity for the older units, while in the newer ones the water pump cavity is separate from the exhaust cavity.


   The manufacturer specifies "On jet drives in which the cooling pump is attached to the top of the bearing housing, it is not possible to provide a flushing inlet.  Immersing the jet drive in a barrel of water doesn't work either, because the water is thrown out.  Using a garbage can or wheelbarrow is not even an option as they will be emptied in about 16 seconds flat, sometimes even before you can turn the motor off.  These units the impeller is turning all the time as it is tied directly to the driveshaft.  The forward / reverse is accomplished by a clamshell on the outlet of the jet exhaust.  Reverse (on the water) is supposed to be a happy combination of diverting part of the water forward.  In reality you will have to play with the shifting lever to try to locate this position, not easily done!


   Backing the boat into some fresh water was the solution.


   "Present production provides a flushing inlet where it is possible to do so, depending on the model.  If the parts book shows a flushing bolt and sealing washer, and your early jet drive does not have this, it can be added.  Where to locate this threaded hole is called out on their engineering drawings.   If in doubt contact the manufacturer."  This is found in the operators manual.


   These conversions can be installed on many of the older units by removing the unit, inspecting it to verifying just where to drill and tap the hole, or re-rout the bypass intake line.  It may take a bit of thinking and understanding the functioning of the unit before this project is undertaken however.  I have converted 2, one was a 1971 40hp Johnson and the other a 1968 50hp Mercury. 


   Both of these were the older A & E series smaller pumps and the modifications were slightly different on each. 



We will cover the E series for the older 50 hp Mercury first.


Here is a "E" series drilled & tapped for a 1/8" street Ell, below this will be a 3/16" hole drilled in connecting to the threaded hole & thru the housing into the cavity under the water pump. Here is the bearing housing installed into the pump housing to check alignment & location to drill the hole thru the body in line with the Ell.  The arrow points to a 3/16" hole drilled below the fitting for water to be diverted into the cavity below the water pump.


   Above you will see a hardware store street Ell, this one was a casting and proved to be too long top to bottom by 1/8".  What was found that worked just right off the shelf was a Weatherhead brand normally sold in an automotive type parts store.  It was made from rectangular brass stock which was shorter both top to bottom and side to side.   The one shown had to be shortened on the side to be the same dimension as the bearing housing so it could be inserted into the body from below, then it also being taller got bent on final assembly.  I scrapped this one and found the Weatherhead brand did not need any alteration and fit perfectly. 


   On this pump housing just in front of where the Ell's top will be located is a part of the aluminum housing that may need to be relieved for clearance.


   It takes careful measuring before drilling to be sure you don't screw up.  Here I placed the Ell so it aligned between the 2 grease fittings  on the side and drilled then into the side with a smaller hole so I could shift it if needed.   It needed to be shifted, so I could insert a short brass 1/8" pipe in and thread into the Ell.  Next after the pipe was tightened, I marked it, remove, it and calculated how much was going to be needed to allow for a nylon washer and just enough to align centered a 3/8" NC stainless nut to be silver soldered to the pipe.  In the photo on the right below you will see the white nylon washer, the 3/8" nut, another washer & a 3/8" stainless bolt that has been cut off so that the threads will just stop inside the nut.


   A 3/16" hole was drilled horizontally thru the bearing housing, connecting to the hole below the Ell's threads.   Now I made a slightly tapered aluminum plug that was driven into this outer hole, being sure it stopped short of the Ells passage hole.  Saw it off on the outside. then  file to match the bearing housing.  This hole diverts the water into the cavity below the water pump intake.


   One thing we found is that since the fitting is for 1/8" pipe, that you may not get the water flow force out the pee hole that you normally would when the motor is running in the water.  There is enough flow to cool the motor however since you would not run the motor at full RPM for flushing anyway.  My cousin was worried so he tied a garbage bag over the jet unit, trying to retian more water for longer operation time.  Well what-ever works was fine with him.


Older E series converted to a flush unit, note that there is no divider between the water pump area & the exhaust area & the water pump is bolted directly to the bearing housing.  Note - this old unit does not have the rubber grease coupler L series showing a divider between  the water pump & exhaust area, & the water pump is bolted above the bearing housing onto the pump housing, making it easier to get water into the water pump for flushing.


  Modifications for the OMC or Johnson 40hp A Series Pumps :   This unit is more compact in that the bearing body under the water pump impeller unit is a lot closer to the top of the jet unit body, so there is not enough room to put an Ell like on the Mercury conversion.  Below is how I did the OMC conversion which is a lot easier than the Mercury.  I had never seen a conversion done before, so am not sure how others do it, but this one works.


    Locate a position near the center of the bearing housing on the left side above the grease fittings.  Measure down about .650" (on the one I did it was .750" but that puts the hole just a little bit too close to the grease fitting hoses).  There appears to be enough metal in the bearing housing to raise it the .100".   Line the drill as square as you can and drill a 5/16" hole thru the jet body.  Go thru enough so that you touch the bearing housing enough to mark your location.   Remove the bearing housing and shaft unit.


    Tap this hole in the jet housing to 3/8" X 16 NC threads.


    Using the marked location, drill a 1/4" hole clear thru the bearing housing and into the cavity under the water pump impeller lower plate.  Now  with a 3/8" drill, spot countersink the outer hole you just drilled in the bearing housing.  What you are now trying to do is to make a mating Vee surface that will block any water from escaping when you insert the plug screw, then later the flush adapter.


    The reason for this, is if you notice there is a 3/16" plus gap between the bearing housing and the jet housing.  In operation, this is filled with exhaust gas and water.  It needs to be separated from from the inner suction water.   So now the plug screw AND the flush adapter needs to be just long enough to go thru the jet housing and bottom out into the Vee you have drilled in the bearing housing.  


    In the photo below my adapter threads are longer than the commercially obtained adapter and there as a O-Ring at the shoulder.  This is to that If I did not get the dimensions exactly right on the threaded and tapered end that the rubber O-Ring would compensate some of the take-up.  The overall length of my threaded end from the shoulder to the end is 5/8".   This could vary on how deep you countersunk the bearing housing however.   The center water hole in the adapter is 3/16".   I just used an old garden hose female end and lathe turned a brass shaft.  Match the taper of the 3/8" drill to the taper you make the plug and adapter tapered end.  The shaft is held into the threaded garden hose end by the rubber garden hose washer.


    The plug screw will be made to the same dimensions & use the O-Ring as the adapter.


Notice the longer threads & the taper on the threaded end


   In the left photo below, you can just see the inner threads of the plug screw at the point of the arrow.  In the right photo again at the arrow is the inner hole into the water pump suction chamber below the impeller plate.


   In final assembly you need to install the stainless steel water pump impeller plate on top of this aluminum bearing housing and the water pump then bolted down on top using the 3 threaded bolt holes shown in the photos.  Place this impeller plate so you can see the shiny wear of the impeller up.  It can only go one way after you have the wear up because there are 3 unevenly spaced screws that will only go one way.


Plug screw  & inner threads shown Inner hole into the water intake chamber


  Alternate OMC Method :  Another method would be similar to the Mercury conversion shown above except no Ell.   Using the same location, drill into the bearing housing and tap it for 1/8" pipe, then open up the jet outer housing so the pipe would be a slide fit and yet thread into the bearing housing.   Cut the pipe and silver solder a 3/8" stainless nut onto the pipe allowing for a nylon sealing washer under the nut.  Then you can use the factory hose flush unit.  This method lets your unit that stays on the motor protrude the thickness of the nut farther out.

   A suggestion on this A series jet units is that when you are reassembling the foot, replace the 1/4" stainless steel Philips head screws with stainless steel  1/4" Allen head screws.  If the Philips heads get worn, your only hope to remove the foot is to hacksaw the heads off for the foot removal.  At least with the Allen head the wrench will not be forced out in the confined space you have to work in.    If the motor has sat for a while and you happen to be the chosen one to work on it, these bolts may well be frozen.


   Also for these older units, they use a stainless steel round pin of about 3/16" diameter to keep the impeller from rotating, plus an eared washer under the shims to keep the pin in place.  They also use a stainless steel cotter pin to hold the impeller nut in place.


Copyright 2007 - 2020  LeeRoy Wisner  All Rights Reserved

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Originally started 02-03-2007, Last Updated   11-09-2020
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