Mercury 4.5hp 1975 - 1985, Single Cylinder, Getting it to Run Again

The motor we will be discussing here is a 4.5 hp single cylinder motor marked on the rear of the cowling as 45.  These single cylinder Mercurys were in production in one form or another since the early-1960's.  They started out as a 3.9 hp in 1964, and were upgraded to 4.5  hp in 1977.  There also appears to have even been a 4 hp version made preceding this 4.5 hp model that was very similar.  I am aware of one 4 hp made in 1969.  There is not a lot if information out there on any of these.

This one was made in 1977 and it appears that this version was marketed as 45 or 4.5.   These were made from 1975 to 1985.  Some were marked 45 while others 4.5 and I have not found a Mercury person who can shed any light on this subject.  

Mercury has a complicated model, year of manufacture system for those of us not in the inner circles of Mercury marine repair.  CLICK HERE for a link to a Mercury serial number guide.    Mercury has no date code, but uses serial numbers to identify the year (somewhat).  These motors started in 1975 with S/N 4108120 thru 1985 with S/N 6608536 at the start of that final year.

Another website that has old Mercury information is HERE.  Here is another that may be of interest for old Mercurys  CLICK HERE.   Or HERE for Older Merc Reference Info.

From the information I can find, these 4.5 hp singles were made in US, Belgium AND Japan, so it is hard for us laymen to positively identify them.  The one I was working on here has electronic ignition, while I am aware of 2 other of these motors in Europe that have magneto ignition and both were newer than this motor.

This little motor being worked on here is a rather sleek unit that has the lower unit angling slightly rearward like Mercury introduced in the late 1950s as a 3.9hp.  It appears to be pretty well designed motor but has a few hidden secrets for disassembly if you do not have a manual.   

This motor was brought to me by one of my cousins that had been setting for about 12 to 15 years and he recently decided to give it to a grand-daughters husband, but after much cranking and probably some cussing he and his son could not get it to even sputter.  There was a lot of fuel laying inside in the lower cowling, which after tearing it apart appeared to be from a bad carburetor strainer bowl gasket.   Boy, this was different for a shade tree mechanic who had not worked on a Mercury since about 1967.   Now for this motor I have violated one of my own rules by not having a service manual.  Take it from me, it can be done as posted here, but not if you value your time.  So all the information in this article was garnered by seat of the pants general mechanical knowledge plus a goodly bit of experimentation and luck.  The choke removal took about a week of thinking and then one night about 2AM the answer came to me.  I almost got out of bed to go out and confirm this.

One word of advise, is that if you find yourself in a situation like this, make good use of a digital camera.  Because by the time you diagnose the problem, tear it apart, even partly apart or wait for the parts to come in, you may well have forgotten where the wires go, how the choke functioned, and a few other important things.

Initial Check Out :  Initially the spark checked out, which was good.   The thought then was to next just pull the carburetor off, then soak it in carburetor cleaner overnight.   Big surprise, with both the carburetor mounting bolts removed, the carburetor could not be taken off as there was not enough room inside the lower cowling because of the binding against the choke and or the front RH upper cowling mount base.  It only took a week or better of sleeping on it to finally figure out how the choke unit had to came apart.  This choke assembly is made of plastic which operates from a lever on the front of the lower cowling that operates a lever inside that operates a pivoting choke plate.  The plate has a egg crate screen that lays over the carburetor for the un-choked position.  The kicker is WITH THE CHOKE STILL IN PLACE the carburetor will not move enough forward to come off the RH manifold stud and it will not rotate enough by being stopped by the upper housing mount boss.

Tricky Choke Connector :  The choke lever is located slightly off center in front on the lower cowling.  When in the run position this lever is in the vertical position.  This system is that the outer choke lever/shaft has a winged key that goes thru the inner lever, then routes into mating inner notches as shown in the RH photo below with the choke unit off the motor.  Not shown is a coil spring that goes between the lever and the cowling to hold things in place.  To separate this choke linkage, you need to use a medium screwdriver held between the carburetor front and the inner plastic choke lever.  Now push the outer choke lever INWARD (under spring tension) as far as it will go, about 3/16" then rotate it to the left 90 degrees.  Now pull the lever forward which disengages it from the inner lever.  Be sure to save the 1/2" long coil spring under the lever and remember where it goes. 

This is the single pivot bolt in the front of the carburetor that goes thru the choke plate, remove this bolt.  Now the tricky part will be for you to observe the location of the flat Y shaped and wire choke tension spring.  This is a snap over type where it's position locations are critical for final, functioning.  If you have a digital camera that can take good close-up photos this would be a time for that.  You can then remove the actual choke plate from the carburetor, lift this plate up and out, now allowing room to remove the carburetor.

Also removing the fuel strainer off the carburetor helps.  Just be sure that the cork gasket that seals this filter canister is intact on reassembly.

One suggestion again is to closely observe the relationship of all of these choke parts, if not, on  reassembly, you may be scratching your head and other parts of your body.  There is a screw into the upper center of the carburetor that the choke plate pivots on.  You will have to preassemble the flat arced tension spring, wire tension spring onto the arm peg thru the choke plate and the screw  BEFORE you replace the choke knob and lever, unless you have very skinny fingers to get the screw into place, then tighten it in.

In the photo below on the left, you will notice the fuel pump connected directly to the carburetor's strainer canister.

Top view of carburetor & choke system in the run position.  Note the grated choke plate covering the carburetor air intake. The 3 main parts of the choke, notice the wings on the outer ever & mating slots on the inner lever.   The spring is not shown

Carburetor Cleaning :  With the carburetor off, it is simple to disassemble it for soaking.   The first thing you will notice if you are more familiar with the OMC along with other brands of motors is that the float bowl is on the RH side of the carburetor instead of underneath.  The fuel enters on the side of a cylindrical pot metal/aluminum casting that has a smaller cylindrical fuel strainer with VERY fine stainless steel screen.  The float is a round black hard plastic material that slides vertically on a center pin.  The float itself raises, putting pressure on a pivoting arm that pushes against another arm that pushes the float needle shut-off valve.

You will probably have to visit a Mercury dealer and purchase new cork gaskets that the fuel strainer body sets into of the carburetor's float bowl top.  However our experience is that you will need to purchase the whole gasket kit #1399-1717 at a price of $8.35.

This carburetor, like most later motors DOES NOT have a adjustable main-jet, but a fixed jet in the bottom of the carburetor.  It does have an adjustable idle jet on the top section.  The usual initial setting is to lightly bottom this needle out, then back it out about 1 turn.  Once you get the motor running and warmed up, with the twist grip throttle set at a slow position, you can slowly screw it in (clockwise) a 1/8 of a rotation at a time until you get it to idle good.  If it starts to run badly, turn the screwdriver the other way again in increments, keep trying until you find the "sweet spot".   This needle screw jet does not protrude, but the head is about flush with the front of the carburetor's housing.  A normal medium small screwdriver is used to adjust it once you get the motor running.  On this particular motor, this setting was 3/4 of a turn out from lightly bottomed.

Once this idle jet screw is adjusted, then you can fine tune adjust the idle stop screw listed below.

It has 2 speed adjustment screws, one for slow speed and the other for high speed.  However these adjustments are not on the carburetor itself, but on a arm attached to the timing plate.  In the photo below, you can see them about 1 1/2" apart with the protruding heads pointing outward.  The rear (RH here) is for the slow setting.  The high speed stop setting is not for carburetor setting, but more to not 'over advance' the timing at WOT, (wide open throttle).

Carburetor speed adjustment screws

Flywheel Removal :  The flywheel may need a puller to remove it.   A three screw type puller needs to be used, however the screw threads in the flywheel are 5/16" x 24 National Fine as compared to 1/4" x 20 National course for OMC motors.  The flywheel nut is a heavy duty 7/16" x 20 National Fine, which uses a 3/4" wrench. 

Electronics :  It appears that these motors were the single cylinder version of a larger twin cylinder model, but here we are working on the single cylinder version.  Information is rather vague but the electronics seem to be somewhat shared between the two.     

This motor that I worked on was a single cylinder and appeared to have electronic ignition and the coil unit is placed in a very convenient location on top and at the rear of the cylinder water jacket as seen in the RH photo below.    Note - I have been in contact with a person in Greece, and England who each have one of these single cylinder motors, according to the serial number the Greek's is a US made in 1983 and the English is a Belgium made in 1985.  Both of these motors have magneto ignition.  Initially I thought the English was an older motor (until I re-contacted the owner) it seems that Mercury used up some of the older excess components for export motors.  But now with the Greek motor surfaces being a US made and dating it earlier, I really don't know what to think.  However there appears to be numerous models and locations of manufacture of this 4.5hp motor, so possibly we have differences.

With the flywheel off this electronic ignition version, there is really nothing you can do to the stator other than clean any possible rust off the magnet coils as seen in the LH photo below.

Top view with the flywheel off & choke removed showing electronic ignition Top view looking rearward showing the coil & wiring.  The green wire is power in & the cream wire is ground.  The black is from the kill button.


Magneto ignition version base plate, also showing the two throttle arm stop screws

Kill Button :  The kill button protrudes from the RH front of the lower cowling as seen in the LH photo above.  It has 2 wires coming from it.  One goes to the ground terminal of the coil & the other is grounded under a side-plate cover screw.  

If someone has played with this button and or it does not kill the motor without concentrated pushing, pull the rubber boot off and inspect the inner contacts.  The 2 contacts in the base are placed on opposite sides and are cone shaped bullets that protrude inward.  Inside the rubber boot is a metal domed hat with a nail head like protrusion on the dome which should point outward.  Inside & centered in the rubber boot is a small hole that accepts this nail head.  With the nail head snapped into the rubber boot, this boot keeps the dome contact retracted from the inner contacts until it is pressed inward.

Shifting Lever & Linkage :  The shift lever is just rearward of the kill button.  This lever and associated linkage inside the lower cowling appears to be made of plastic. 

The linkage from the lever unit goes to the center and under the carburetor that is connected to a rotating shaft that goes down into the exhaust housing then into the gearbox.  The shifting shaft in the gearcase has splines on the top as seen in the photos below in the water pump repair section.  These splines match into the upper shaft.  There is one spline tooth missing on the rear of the these mating shafts so that it is impossible to get the shifting out of synchronization.

This shifting shaft can be seen in front of the driveshaft in the photos below in the water pump repair section.

Bore :  The bore of this single cylinder motor is 2" which uses two rings, from the appearance they appear to be about 3/32 wide.

Recoil Starter :  The Recoil rope starter is located in the upper cowling.  This uses a very efficient expanding dog system when the rope is pulled.  You probably will never have to remove it unless the rope finally frays out and breaks but this should be rare on this model as the rope is guided by a roller as it exits the spool then goes out thru the cowling (the normal wear spot).   It is accessed from one Phillips screw from the top of the upper cowling.  When replacing the rope, be careful of the recoil spring coming unglued and jumping out at you like a vicious snake.

One problem with this motor is that it is almost impossible to have the motor running with the cowling off if you try to adjust the carburetor's idle screw.  This is because with the recoil starter unit being made into the upper motor cowling.   And there is no provision on the flywheel for an emergency rope start.   It is hard to remove the cowling with the motor running without the flywheel rubbing the cowling because of the tight fitting cowling around the flywheel.  

The best way may be to remove the 2 side latches inside the cowl before you try to start it (2 bolts shown on each latch below).  After doing this, you will have to hold down on the cowling firmly while you pull the rope to start it this way.   It is then easy to lift off the cowl after you've got the motor started, otherwise you may get some flywheel rubbing these cowling mounts.  Adjust your idle jet, then shut it down and re-install the latches.  

Or use a electric drill motor and a socket to fit the flywheel nut.

Shown here is the recoil starter which is mounted in the top of the upper cowling.

Spark Plug : The normal spark plug will be a Champion #UL J4.

Fuel Pump :  The fuel pump is attached directly to the filter system on top of the carburetor by a short threaded neoprene fuel line.  This pump appears to be an overkill in this motor, but I an assume they are using the same fuel pump on many other size of motor in the Mercury line.  They appear to be a very sturdy pump with minimal problems.

This fuel pump is activated by vacuum thru a rubber hose tapped directly into the RH side of the block.

The fuel pump is shown here in the center right, directly attached to the carburetor, also showing the vacuum line to the block.

Fuel Line Connector :  The fuel line connectors are the old standard Mercury bayonet type, with the one on the motor recessed in the front of the motor's lower cowling.  This location is very protected.

Fuel Mix :  The fuel gasoline to oil mix should be the normal 50-1 of 87 octane gasoline and TCW-3 outboard marine oil.

Upper Cowling Latches :  These latch levers are located just forward of the middle and on the sides of the lower cowling.  To unlock, they need to be rotated forward, which rotates the bayonet shaft from the upper cowling and allows you to lift the upper cowling straight up then off. 

Throttle Linkage :  The throttle handle shaft goes to a bevel gear system inside the lower cowling, that then is connected to the timing plate.  If you remove the 2 opposing Phillips headed screws at the base of the tiller handle, be sure to not loose the large flat washer and nut associated with them.  These are used to put tension on the tiller handle so it does not flop up and down.

Mercury engineers did a pretty good thing when they designed this twist grip throttle.  It has the meshing knobby teeth on both the tiller handle and the intermediate shaft leading to the timing plate bevel gear.  But there is no problem of getting these teeth out of time because the tiller handle shaft is 1/4" square with threads on the outer end allowing the twist handle to be screwed onto the shaft.  Back a ways from the outer end & behind the threaded portion, is a round section turned into this square shaft.   When you get the twist handle screwed on so it abuts against the tiller handle abutment, the timing plate is in the proper location, you simply tighten both opposing set screws into this turned round section, locking it in place. 

The timing part is to rotate the timing plate so that the throttle arm is just starting to be pushed forward.  This should be near where you now want to align the start indicator on the twist handle with the raised indicator on the tiller handle.  If it id not exactly right, then loosen the 2 set screws & change the placement of the throttle arm against the timing cam, retighten the 2 set screws.

Tiller Handle :  The tiller handle pivots in the tiller base.  It is secured by a screw on each side and has a unique washer that has protrusions on the outside that are threaded to match the screws.  There is a thin stainless steel wafer washer on each side also.  The handle pivot on this motor was so loose that it could not be made to stay in any slight upward position.  I took the stainless washers and copied them onto the sides of margarine tub lid, cut them out with scissors, making a friction type spacer washer tightening up this floppiness.

Motor Mounts :  There are 2 opposing rubber motor mounts on each side of the clamp bracket (2 top and 2 bottom) that attach this bracket to the exhaust housing leg.  These mounts are a simple 1.250 " round aluminum slug that is rubber bonded to another slug 1.050" with the whole thing is .850" thick.  The smaller end goes into the 1.250"daimeter hole in the clamp bracket first and into a recess in the exhaust housing.  The larger end slides into the bracket's hole enough to be held in place by a snap ring.  In the center outside of this mount is a threaded 10-24 hole and it's mating screw holds a plastic cover in place covering the mount's outer end and snap ring.

One word of advise, UNLESS you have to remove these mounts, it may be best to resist the temptation to remove any of them just to see how they function.  Reassembling them back into the inner recess, everything needs to be in perfect alignment, and if the outer bracket may have gotten sprung in any way, you may be in for a experience.  You may need to use a large C clamp to put pressure on both sides at the same time in order to get the 2nd snap ring into the retainer groove.

The reason for the above bit of advise is that this motor at one time must have hit a underwater object going fast enough to break the motor mount at one of the lower area.  I ground metal out, cleaned it up, clamped it in place, then had it Heliarc welded.  The rubber mount was not placed in until after the welding.  Either something warped, or there was a lot of tension initially as trying to get these back in then snapped in place was quite a chore.

Lower motor mounts behind the round cover

Shallow Water Usage :  In the front RH side of the tilt shaft area is a small plastic lever that rotates up and down.  This lever activates a bracket in and at the lower edge of the outer mounting brackets.  When lowered you can tilt the motor slightly forward, this bracket snaps up holding the midsection/lower unit up slightly for use in shallow water.

Fresh Water Flush :  Immediately below the tell tale water outlet of the motor, tucked into the lower cowling is a dime sized screw headed plug screw.  There is embossed on the cowling "Fresh Water Flush" near this plug screw.  It has a 3/8" course thread now commonly used by most all motors that use this type of a flushing attachment.  Usually these flush holes are not to be used for running the motor out of water or a tank, but to flush it without the motor running after it has been subject to a salt water or muddy silt laden environment.

Water Pump Repair :  To remove the lower unit, there are 2 nuts that need to be removed.  One is obvious, being exposed in the front of the exhaust housing above the lower unit which uses a 9/16" wrench.  The other is buried up under the cavitation plate and needs a 1/2" socket for it's removal.  Once these 2 nuts are removed the lower unit can being pulled downward.  The front upper nut does not have enough room to be removed completely until the lower unit is dropped about 1/8".  If there is any debris ore slight corrosion on the studs the nuts were removed from, you may have to place a large screwdriver in on top of the front upper nut to force the shaft downward initially.  You can tap the rear section of the lower unit with a plastic mallet to help it come of straight down.

Once the lower unit is off, be sure to not loose the water pump outlet plastic guide tube that is 9/16" outside diameter and about 3" long.  This just sets into the pump outlet and is essentially just a guide for alignment of the water pump outlet to the copper tube in the exhaust housing that goes up into the block.   It is not shown in the LH photo below, but goes into the large raised boss just behind the driveshaft.  Remove the 2 upper nuts on the water pump housing and the top can be slid up and off the driveshaft.  There is a stainless steel plate that just sets in the bottom of this upper housing that needs to go back in place above the impeller when you reassemble it.  It can only go one way as on the upper side of this plate is a protruding lip that needs to go into a mating slot in the upper housing you just took off.  You can also tell which side does down by the wear pattern of the impeller and the indents in the bodies. 

Replacement impellers can be a pain trying to find the right one as some look alike but may not be compatible.  The outside diameter is the same, but the thickness may be different.  If you use the thinner one in place of the thicker one, the pump will not be efficient, but if the new impeller is too thick, it will be so tight that on the initial start up, you will have a ruined impeller plus possibly both the top and lower pump housing.  Also if the water pump unit was changed to a newer style, it can use a different impeller than the original.  And if that is the case you have no way of knowing.  There are at least 3 look alikes for this model.  So on this motor, if you are not sure, order a new water pump assembly complete.  

If you decide to go for a complete water pump, I offer a bit of advise.  The lower pump body has a O-Ring seal in the bottom of it that seals the lower unit oil in above the shaft ball bearing.  After you remove the lower main pump body, do not lay the lower unit on it's side otherwise gear oil will run out past the shaft's ball bearing.

Clean the surface above the bearing so that the new pump body sets tightly to the base metal, while being sure to not let any debris get into this bearing.  Replace the water inlet tube's grommet in the lower pump body.  The other tube grommet will usually stay inside the lower unit hole, if not, then clean and replace it.  Both of these grommets have a 1/8" round ear that holds them in place.  Clean the tube on both ends.  Insert the tube into the lower housing grommet, as the lower housing is pressed downward, also push the bent tube end into the grommet in the lower unit housing.  In all likelihood, you will not be able to push this body down enough to bottom out on the base because of the O-Ring's restriction, so slide the upper pump housing on (without the impeller) then tighten both down  using the 2 nuts in the lower unit housing.  Once it is bottomed out, remove the nuts and the upper housing and then proceed installing the impeller and the upper plate.

The standard replacement impeller #474-89981 costs $17.60 as of the date of this article.  It has 6 vanes, new unused dimensions of 1.250" in diameter, .466 thick & has a shaft hole diameter of .460.  The water pump kits come in 2 versions, (1) without the impeller & (2) with the impeller & both upper & lower bodies, grommet, an O-ring & plastic washer that I can not find where they go is.  The (1) stripped unit is  #46-70941A-1 and the (2) complete kit #46-70941A-3, which sells for near $60.00.

Replacing this impeller into the lower stainless steel cage of the water pump housing can be a bear, as you need to be sure that the hat shaped impeller key stays in the driveshaft slot as you slide the impeller down on it while forcing the impeller vanes in the direction they will be when the motor is running, (clockwise looking from the top down).  It can be hard to align the impeller key as you slide the impeller down the driveshaft and at the same time try to push as many of the vanes inside the stainless steel impeller housing all at the same time.  Many times if the impeller and the key are not in perfect alignment the key may twist sideways before the impeller is totally down into the cage.  What I found that helps align this impeller key is to file the top of the key to a slight taper.  This allows for some slight initial misalignment by letting the impeller self-align.  Also you might put a small amount of chassis grease on the mating surfaces of the key to the driveshaft flat to hold it in place while you try to get the impeller in place.

Make note of the location of the water pump outlet of the upper part of the water pump housing in relationship to the water pump itself.  It can be bolted on 2 ways, but will only fit in the exhaust housing to mate with the copper tube to the power head one way.  The upper pump body and the lower body have a flat spot near one of the retainer bolt holes, align these 2 flats together.  The water outlet tube goes on rearward of the driveshaft as shown in the photos below in the gearcase section.

Also be sure that the water supply tube to the water pump is free of debris.  If this tube is plugged, then you have just ruined a new replacement impeller, and very possibly the pump housing both top & bottom also because if it does not pump enough water, the pump get so hot it can melt & ruin the plastic housings.

In the RH photo below someone apparently did not install the last impeller properly as of the 6 vanes, only 2 were in place for proper rotation.  This may have functioned for this low power motor that was only used for trolling, but when it was removed these vanes were so old and hardened that they had taken a set so bad that they would have never have contacted the edge of the inside of the impeller cage to be as efficient as designed.

This old impeller may have been installed incorrectly, notice some vanes are pointing each direction On the bottom is a new impeller with the old one on the top.

Water Pump Inlet :  This is a stainless steel slotted screen about the size of a half dollar coin on the bottom part of the cavitation plate directly in front of the propeller that one of the lower unit retainer bolts goes thru.

Water Passage Thru Motor :  The water is pushed up from the water pump thru a 3/8 OD copper tube to the bottom engine cowling where it sets into a 1/2" dia. shallow recess that has a 1/8" plastic seal where the water passage goes into the block.  If you replace the water pump impeller but do not get adequate or irregular cooling water pot the pee hole, you will need to remove the power head from the mid section housing to remove any debris at this tube juncture or even drill out the plastic O-Ring if it has gotten hot enough to deform this washer/seal.  This location as seen in the photo below at the arrow.

This photo below has the lower pan separated about 3" from the power head just enough to show the water tube under the power head.   The fuel connector has been disconnected from the lower cowling.  To separate it further the wiring would need to be removed from their terminals.

If you have to go this far, with it apart, there is one simple thing I would do and that is to drill a small (3/16") hole in the bottom cowling under the carburetor.  It seems that this motor can accumulate gasoline in the pan of the lower cowling then it can not run out.

Power head separated from the lower cowling & midsection showing the washer & water supply tube leading to it

This motor does not use a thermostat.  The water is pumped directly into the lower water jacket and around the cylinder walls.  There is no cylinder head on this motor as the cylinder and head are cast as one out of steel.  The rear cover of the block can be removed, but it does not allow you into the cylinder, merely into a water jacket around the outside of the cylinder walls.  This rear cover has a hole that the spark plug protrudes from.  The cover has a gasket around the spark plug which seals up to and around the base of the spark plug.

The pee water comes off the top of the block after it has cooled the engine.  The pee hole water is normally not very hot as this motor does not put out a lot of heat in relationship to the amount of cooling water even when running fast.  So on this model, if there is NO WATER coming out of the pee hole, your water pump system is not working.  Part of the water from the water jackets is also diverted down and into the exhaust housing to mix with the exhaust to cool this mid housing.

Reassemble Powerhead to Midsection :  The upper shifting shaft has external splines on both ends, the upper meshes in a bevel gear that is connected to the lever shaft.  The lower splines mesh with the internal splines of a coupler that in turn meshes with the splined shaft in the lower unit.  These splines are made so that one spline is missing.  This will only allow things to go together one way.

In reassembly the shifting shaft has to be installed in the midsection first before you try to set the powerhead back in place.  This shaft can not be inserted from below.

The shifting lever can not be gotten out of time as there are only the right number of teeth on both bevel gears.   When reassembling, dropping the powerhead down, align the shifting shaft bevel gears onto the shaft, slide things down which allowsthe shifting lever detent spring to slide into position. 

You now will have just enough room to re-insert the fuel line coupler & reinstall the retainer bolt.

Reattach any electrical wiring that you had removed.

Slide the powerhead / lower cowling down enough to replace the (/6) 4" SAE nuts holding it to the midsection.

Gearcase :  To check the oil in the gearcase, remove both the large plug shown at the RED arrow, then the small plug screw at the BLUE arrow.  Oil should come out the lower plug hole.  The lower hole is for draining and filling this gearcase while the upper hole is a vent/oil level indicator hole.  It is best to use a recommended oil that it can be pumped in using a gear oil container with a plastic push pump and hose located in it's top.  Pump new oil in until it starts coming out the upper hole and quickly replace the plug screws.  Replace the upper screw first so that it helps create a vacuum, helping to retain some of the  gear oil as you try to get the lower plug screw started in the threads.

As mentioned above in the water pump section, note that the water pump outlet is rearward of the driveshaft.

Water pump assembly still on the lower unit.  Notice the splines on top of the shifting rod shaft Drain/fill plug screw in RED with full overflow in BLUE on top

Reassemble Lower Unit to Exhaust Housing :  When reassembling, be sure that you apply some marine grade grease to both the driveshaft and shifting shaft splines to insure a easy disassembly next time.  Also be sure that the plastic water pump tube guide is in place in top of the water pump housing. 

The lower shifting shaft has external splines that mesh with the internal splines of a coupler that in turn meshes in a bevel gear that is connected to the lever shaft.  These splines are made so that one spline is missing.  This will only allow things to go together one way.

In reassembly using pliers, rotate the shifting shaft of the lower unit as far counterclockwise as it will go, which is forward.  Now move the shifting lever also into forward. 

Using a flashlight, slide the lower unit up with outlet plastic tube onto the copper tube inside the housing, while at the same time align the driveshaft into the extension that guides the shaft into the crankshaft splines.   If you get the shafts started, but they do not want to go completely for the last 1/2", you may well have to rotate the flywheel slightly and also possibly move the shifting lever minutely to get these splines to engage.  The front 3/8" stainless nut may have to be placed onto the shaft before it is completely up into place as there is not enough room to start it if the other 4 bolts are tightened.  The bottom, rear nut can then be tightened down so that both nuts pull the lower unit up evenly to the exhaust housing.

Propeller :  The propeller is a thru the hub exhaust 2 blade  8" diameter with a pitch of about 8 or 9" (rather hard to measure the pitch on these things).   A  3/8" (9/16" wrench) plastic dome covered stainless steel nut holds in a thick splined washer on the propeller shaft.   There is no shear pin as this prop uses the rubber mounting hub. 

This propeller had not been off for about 15 years and had been used in brackish salt water of a bay.  You can guess that it was seized to the shaft.  Numerous days of setting it so the exhaust was UP and keeping penetrating oil around the splines helped in removing this propeller.   When replacing the propeller, apply some marine grease onto the prop shaft splines if you want it to come off easily the next time.

Aftermarket Spare Parts :  You will not find many aftermarket spare parts for this motor in the NAPA or Sierra catalogs.  They just don't go that small in the Mercury motor line of aftermarket parts. 

This pushes you to a marine dealer who has access to Mercury spare parts.   One local Mercury dealer is not the most jovial person around, especially with the declining economy and since he is now the only mechanic there added to the fact that his wife and parts person passed away with cancer a few years before.   He now is also the counter person so when you interrupt him from doing repairs in the back for a minimal parts purchase, well you may have already guessed the outcome.

A online marine parts store that has proved beneficial to me is  They offer original manufacturer parts at a discounted price below retail, offer a phone salesperson if need be and do ship Fed-Ex to your door.  They have exploded factory parts online, however it may take a bit of doing trying to initially understand the factory descriptions of different sub-sections.

Observations :  All the above information was derived by seat of the pants trial and error, (which I normally do not recommend) no manual was used.  Here the repairs were done, or tried only to be taken back apart numerous times, added a procedure then redone if needed.

I do not like the situation where you can not start a motor with the upper cowling off so you can make final carburetor adjustments without having the flywheel rattle against the cowling if you try to remove it while running without disassembling the latches.  Also the inability to positively identify the water pump impeller without having many at your disposal so you can compare is a hindrance.  

Most all marine dealers will not exchange impellers if the plastic bag has been opened.   I can somewhat understand where the possibility exists of someone swapping a used impeller back into the bag then wanting a refund.  But this sure makes things bad for the average honest guy.   It kind of seems that they want you to bring the motor in to them for any repairs but look down on those of us who do our own work.

These Mercury motors design engineers have their own methods needed as to tear apart and re-assemble.  Not that I am saying there is a right or wrong, but they are sure manufactured different than the old OMCs that this country boy is used to working on.   And it has been said that Carl Kiekhaefer was either a mechanical genius, or an eccentric who had strange ideas, some that were short lived.   Sometimes it is hard to tell a genius from a insane person, as they frequently waver from one side of the line to the other. 

However, if you go to and look up replacement parts for these older Mercurys, IF the part is available, consider having your heart medicine or a bottle of Jim Beam handy.

Copyright 2010 - 2018  LeeRoy Wisner  All Rights Reserve

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Originated 03-21-2010, Last updated 08-31-2018
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