Maintaining Johnson/Evinrude
4hp  Outboards

1969 - 1978






OMC (Johnson and Evinrude) utilized a lot of parts interchange between models and HP of closely associated motors in those days as it was simple to create "Families" of near HP size motors using the same parts, which was a great cost saving idea.  Here we will usually be many of the 4 hp parts be shared downward with the 3 hp motors, but not upwards with the 4.5 hp or larger.

General Observances :  The production of this series of 4 hp motors ran from 1969 to 2001, BUT there were many changes during this time.   The motor started with magneto system (points and condensers), then to electronic ignition being introduced in 1977.   The1978s and newer had a different fixed main-jet carburetor.  And then the later version starting in 1979 a totally different starter mounted in the new upper cowling and was used, and at that same time the remote fuel tank was abandoned, with a fuel tank in the upper cowling.  At this same time the carburetor was changed somewhat by adding a fuel shut off in the carb necessitated by the cowling mounted tank.   On this article we will cover the first year of production version.


The most noticeable part of this motor is that what you would normally think the right side lever would be a shifting lever (WRONG) it is a throttle lever.  The earlier 5hp had this throttle lever on the left side.  This motor does not have a shiftable gearbox, so it is in gear all the time.  Reverse is obtained by rotating the motor 180 degrees.  This cuts down on weight and cost.  It was designed as a fishing, (trolling) motor for small boats, and it did an admirable job at it for a decent price.  It was made to use a remote fuel tank, which was rather new for a motor of that size.  It was made with a standard lower unit OR a Weedless version.  The difference was the standard propeller shaft was at 90 degrees to the motor driveshaft housing, while the weedless prop shaft sat at a downward angle.


It is my opinion that they were designed to compete directly with the Eska and Clinton motors being sold by Sears that at that time were using basically 2 cycle air cooled lawnmower motors.  These air cooled outboard motors did not idle down anywhere close to what the water cooled name brand motors did, but being cheaper, they cut in to the name brand sales.  Some first time fishermen did not really know what they needed, and purchased through a catalog or non fisherperson sale staff, but over time, may have found the error of their ways.


This 4 hp motor does not follow the normal line of progression as far as to yearly manufacturing, therefore it is my guess that they (initially at least) reverted back and cleaned up the warehouse using discontinued/obsolete parts and simplified (or tried to) it as much as possible.  By this, it appears the starter was from the earlier 3hp and the carburetor also reverted back to the dual adjustable idle AND high speed knobs.  But the fuel pump was the same small square one as used on the other motors made at this same time. 


All three of the main bearings are simply bronze bushings cast into the aluminum block, and the connecting rod bearings are again but simplyer just reamed in the aluminum connecting rod.  This requiring a richer fuel/oil mix than other OMC motors made at the same time frame.


Upper Motor Cowling :   This cowling is a slightly miniaturized version (shortened by about 1") from the 5 and 6hp models made during closely related times.  But the latch was combination of others models, but closer to the 6hp of the same time period.



Magneto Ignition, Points & Condensers :   The ignition is magneto type, using points and condensers.   Points were universal from the 1.5 hp up to 30 hp from 1952 to 1976 and then even to some 40 hp motors to 1973.  Condensers pretty much followed this pattern from the 1.5 hp to the 35 hp at 1976 (excluding the 9.9/15 which are different).


On these motors, all the points are set at .020".   If you get a motor that is not firing, pull the flywheel then clean up the points with a point file.   The last point file I purchased (2009) cost $7.50 from a auto parts dealer.  Sometimes all you are doing is to just remove oxidation off the contacts if the motor has sat for some time.   This can be usually remedied by running a point file thru the point openings to freshen up the contact metal.  Most of the service manuals say to replace the points if worn or burned.  Well, take it from me you can usually clean them up for many more hours of run time afterwards.

However if the points are really pitted, that is a sign that the condenser is not performing to capacity.  If this is the case, then it is recommended to replace them, along with the condenser.  The one thing that does wear is the points rub bar if it has not been lubricated from the last repair.   


If you replace the points and still have running problems, you may well could have contaminated the point surfaces with oily fingers.   If this may be the case, remove the flywheel, rotate the crankshaft until one of the points are open, wash it off with lacquer thinner, rotate and do the other set.  Then insert a section of brown paper shopping bag in the points, rotate the crankshaft until the points close then pull this paper thru the points.  This should clean them.


This older ignition using condensers, if they start to break down, the motor can start and run for a while, but when things warm up, the condensers and or coils can begin to short out internally causing the motor may die.  You can crank until you are blue in the face but it refuses to start.  Let it set for an hour to let these components to cool down and it MAY then restart.  Time to change condensers, however hang onto the old ones as long as possible as the newer manufactured ones have a reputation of not being made that good.

A condenser is simply a lot of aluminum foil wrapped between waxed paper as an insulator.  If the paper starts to deteriorate, after running for a while, the metal will heat up then expand.  IF this happens in a location where the insulation is bad the metal shorts out to the next wrap, nullifying or shorting out to ground so no output is achieved.  But IF this is the case, many times when the condenser cools down, the short goes away until it warms up again.  Once it has done this process a few times, then the short becomes rather permanent and the condenser can become inoperative

If that driver coil (under the flywheel) starts to fail, is cracked, or starting to melt down, or whatever, there is a possibility that it would also operate when cold, but fail by shorting out when heated up by the running of the motor.   However do not just replace them because they are oozing black tar like goo.  They may still run for some time.


For a more in depth article on magneto ignition along with fine tuning the point settings  CLICK HERE.  This article is directed mainly at the 1974-1976  9.9/15hp OMCs, but the principal is the same for about all of the magneto ignition OMCs even with those that have external coils.


At the top of the photo below you will see a white pivoting arm, this is the throttle friction system that works against serrations on the lower outside of the timing plate.



Top view of the timing plate with both the points, condensers & coils


Coils :   Coils for these points and condenser motors are the same from 1949 mostly up thru 1973 from motors 5.5 hp thru 40 hp.  This motor's coils are mounted on the timing  plate under the flywheel.  These coils, especially those in the 1960's motors have a reputation for cracking and allowing moisture in and then starting to break down, creating problems.  The factory (or supplier) changed the plastic formulation on the coils at some stage and the replacement ones are considerably better.    If the coils have cracks that you can see, they maybe also have cracks below which could be allowing electricity to short out to the plate.  If you are on a tight budget, remove the coils, dry them out, then paint the cracks with a thick paint or epoxy, which may get you by.

About 60% of the older motors I have had, needed one or more coils replaced, and the current replacement costs range from $20 to $25 each, which would be financially unviable for someone not working on their own old motor with the price of labor at $85 an hour to not replace them.  But the mechanic is covering his behind, because of you possibly later having one/them fail and he would get the motor back for free repair only to find that in his saving you money he actually got an unhappy customer and him a repair where he felt he had to cover at least part of the second repair.


After doing some web surfing I found a website by Doug Penn where he showed on a video how to test both the coil and condenser of older OMC outboards.  CLICK HERE for a coil testing video, and  HERE for a condenser testing video.  


Recommended Spark Plugs :  The recommended spark plugs for these 4 and 5hp motors were Champion J4J, however if you are using it for a lot of trolling, I go to the hotter J6J or hotter yet the J8Js.   The recommended spark plug gap is set at .030", however with a well used motor where the rings may be worn and may oil foul the spark plugs, old some mechanics may recommend try to open the plug gap up from the normal to .035" or even .045" which seems to allow the plug to run hotter and burn cleaner.  However maybe a better solution would be to a move to a hotter plug, like the J6C, especially when doing a lot of trolling.


Carburetor :  To remove the carburetor, you will need to disconnect the fuel line, the choke lever and low speed knob (sometimes called the idle jet).  This Rich-Lean knob just pulls off the finely splined shaft straight forward.  After that, you will have to then remove the two top bolts (using a 5/16" box wrench) from the recoil starter spool mounting plate.  Very carefully pull the complete starter to one side just so far that do not pull the recoil spring out of the base.   You have to remove the starter to access the LH carburetor mounting nut and remove that nut.   Replace the starter unit then bolt it back in to keep the spool and the spring from coming a nightmare.  Remove the RH carburetor nut, slide the carburetor forward off mounting studs. 


There is a small coil spring attached to the carb roller arm and the other end goes in a small hole in a cast in stud on the top of the motor block up under the flywheel.  You need to use a needle nose pliers to unhook this spring, and again to reattach it.

The carburetor is the basic simple standard unit and possibly the last to use the adjustable main-jet style.   There is no carburetor cam roller, but simply the carb arm (#26) rubs on the timing plate cam.

Number 15 in the illustration below is the idle adjustment needle, while #23 is the high speed adjustment needle.  Number 9 being the choke lever.



Early,  adjustable main-jet Carburetor



Throttle Control :   The throttle control lever is located on the right side of the motor and raises about 6" up on the outside of the cowling.  The Fast, Start, Slow indicators are a decal on the lower cowling behind the lever.


It has a rubber friction tube inside the lower cowling supporting this lever shaft and tension adjusted simply by squeezing the cover for the rubber, using it's 2 mounting screws.   Inside the cowling and next to the block his inner part of the throttle arm is then attached to the timing plate by a arm and wire rod, which in turn moves the throttle cam to the carburetor.  The speed indicator decals are placed on the top part of the lower cowling behind the lever/arm.  There is no Kill button, as with the lever all the way back is enough to change the timing to where it will die, if not then choke it.


Here you can see the throttle lever  & the speed indication decals on the RIGHT side of the motor, (NO TWIST GRIP THROTTLE)


Recoil Starter :  This recoil starter uses a unique vertical pivoting gear that when the rope was pulled, the gear unit pivoted up engaging the flywheel ring gear teeth which are on the under side, not outside of the flywheel.  This version was later used on the 4.5 and 7.5hp motors.


Exploded view of the 4 hp starter unit.



Motor Mounts :   There are no real motor mounts on this small motor as the rotating pivot shaft takes up all the thrust and with this small a motor there is not a lot of thrust on the mounting clamps.


Tilt Lock :   This motor has no actual tilt lock, but just a fiction fit on the tilt hinge bolt.


 Motor Bearings :  The crankshaft bearings are cast in bronze bushings (no rollers) and the connecting rod bearings are simply a bored and reamed hole in the aluminum rod, very similar to the Briggs & Stratton lawnmower bearings.  This aluminum bearing being mandatory that this motor use a richer fuel/oil mix than currently recommended for the modern needle/roller bearings.


Lower Crankshaft Seal :  One thing that seemed odd when I into into this project, is that there was no lower crankshaft seal at the block nor attached to the upper driveshaft like the 5, 6 and 8hp models.  Below the lower crankshaft bearing, in the block, is a groove below it for what seemed like may have been designed to accept a seal, but upon investigating, this recess was apparently just for a upward protruding ring on the upper part of the clamp bracket/ exhaust housing that was for block alignment.  So maybe with this motor requiring a richer fuel mixture, that in itself, being more oil settled at the lower part of the crankcase, may have been enough in this small a motor to seal it.  ??


Thermostat :  These motors do not use a thermostat, probably, (1) to cut costs and (2) since it is a small enough motor, they possibly then designed it so the size of the water passages through the head gasket are enough to balance between it being warm enough for full combustion of the fuel AND yet enough flow to keep from overheating.


Water Pump :   To remove the lower unit/gearbox, there are 4 bolts at the lower part of the exhaust housing that attach the gearbox to this housing.  Remove them.  The gearbox and driveshaft should now slide down and off the water pump outlet tube that is attached to the bottom side of the powerhead.   Do not try to carry this unit by only lifting on the exposed driveshaft as it will pull out of the gearbox.  You can then remove the two screws holding the upper water pump housing, which allows this housing to be slid up the driveshaft, exposing the impeller.


This water pump does not use a upper shaft seal, but relies on a wider band of rubber on the impeller to seal around the shaft against the pump's housing on top and wear plate on the bottom.


The water outlet tube from the water pump up to the block is 1/4" copper tubing.



Here the water pump impeller is shown



The early motors used a pot metal water pump housing (now obsolete) and the impellers used on this motor up until 1975  were OEM impeller #0277181,  or if aftermarket, Sierra #18-3001.   If the motor (or if the water pump was replaced) uses the later plastic pump assembly OEM #0379770 (introduced in (1975) then use impeller OEM #0763735 instead.   The impeller key is made similar to a Tee, but the single protrusion is round going into a hole in the driveshaft, where the to of the Tee is square, fitting into the impeller keyway.  This impeller has six vanes and dimensions are approximately .50" thick, 2.74" diameter, shaft hole ID .440"


Water Pump Re-Assembly :   I have found it best to inset the impeller into the housing with the vanes rotated as if the motor was being rotated clockwise looking from the top.  I usually lubricate the vanes with a slight amount of oil or Vaseline.   Place the impeller in/on the water pump housing, being sure you install the impeller in the same rotation that the motor will be turning, RIGHT HAND looking down.  Rotate the impeller as you force it into the housing.   Move the impeller so that it's hole in the hub is pretty much in line with the housing shaft hole.  All vanes need to point the same direction.  Install the impeller key #13 in the driveshaft.  As mentioned above this model's key are made in the from of a Tee.  The bottom peg is rounded that goes into a hole in the driveshaft, leaving the top of the tee which is rectangular slide into the square notch in the impeller. 


Slide the water pump housing (with the impeller down in place) onto the driveshaft, moving it enough to get the impeller and housing to both accept the driveshaft.   Be sure the impeller key in the shaft retainer hole has the hex part of the key vertical to be able to accept it's mating slot in the impeller hub.   Slide the housing farther down onto the key.   It wants to turn sideways slightly enough during assembly that you need to have it rotated very close as the 2 slide together to assure it is right.  The water tube outlet will be to the rear.  Bolt the housing down to the lower unit base using anti-seize on the threads.


One thing when reassembling anything on the lower unit that MAY get exposed to water (especially salt water) is to coat all the bolt/screw threads with a anti-seize compound.  A later mechanic will thank you.  As this motor all four of the lower unit fastening bolts to the exhaust housing were corroded enough that they all twisted off, requiring more tedious work to remove.  As viewed by CLICKING HERE.


The above also applies to the upper driveshaft spline, grease it with a good grade of waterproof boat trailer axle grease.  This will ensure that the next repairman will be able to remove this lower unit/gearbox when he needs to replace a water pump impeller.


Gearbox :   There is no gear shifting, so just a simple set of bevel gears are in a modular cast gearbox with a removable rear propshaft bearing plate.  This 4hp appears to use the same BASIC gearbox as the previous short production run of the 5hp as seen in the photo below.


To remove this lower unit (gearbox) from the exhaust housing, remove the four 1/4" size (3/8" head size) bolts from the bottom side of this unit where it bolts to the exhaust housing.  WARNING, in all probability these bolts will be seized and if you unknowingly think they are starting to unscrew, THEY WILL BREAK OFF.  Looking at the lower part of the exhaust housing above where these bolts are located, you may see some slight holes.  This will be the location of the bolts.  It may be best to soak these recesses with a good grade of penetrating oil for a week or so, hoping to elevate the chance of any bolt breaking.   If not then you will be in for the task to remove these broken bolts, while saving any threads in the housing, (Not An Easy Chore).


 Seen in the photo below are two silver colored dime size threaded in plugs, with the words Oil Level above the upper one.   The other "Fill" plug is on the lower edge of the radiused gearbox.  You remove the upper one, then remove the lower, quickly insert the gear oil fill nozzle and squeeze oil in until it runs out the top hole, which is the proper amout.



Side view of the lower unit/gearbox showing the water pump setting on top and the water intake slots 



Now again looking at the photo below, line up the part you can see of the driveshaft above the water pump, and visually extend it into the gearbox.  It comes out nearer the front than the rear, and to make it as simple as possible, the driven gear on the propshaft is rearward of the drive gear on the driveshaft.  This would, in normal shifting type gearboxes be the reverse gear that is being driven, which is not used here.  Therefore these props have Left Hand blades amd turn backwards from the normal ones. 


Propeller :  The prop uses a shear pin 1/8" (.125") diameter X 1.00" Overall Length.  These pins appear to pretty substantial and I doubt if they would break before a blade on the prop would.  This motor uses a 3 blade aluminum prop in  8" X 5 1/2" size and has a 1/2" prop shaft hole.  I can not find a part number on it, only the size.   The hub diameter is larger than normal for this size a prop because of the rubber clutch.


 These props had the drive/shear pin forward of the rubber drive hub, with a threaded hex nut on the outside rear.   This prop nut is a 7/16" X 20 NC thread and has a circular groove in the center of the hex where the cotter pin can be pinched over and into.   This appears to be the same as used on the early 5hp motors.  Forward side of the nut's hex is a 1/8" turned down section for the rubber cap/nose cone to snap over the complete nut and cotter pin to retain it.  This rubber nose cone has to be pried off rearward and if it has not been off for some time the rubber may become slightly torn.


They made a slight improvement over the 5hp prop, in that this 4hp, which incorporated a "clutch" #28 (OEM #0310331) in the illustrated photo below.  It is simply a rubber double cog ring (cogs on the inside AND on the outside).  As seen below, it fits snuggly inside the prop, and the prop clutch hub goes inside of it, creating a connection.  But over time, this rubber can deteriorate, allowing the prop to slip at a higher RPM. 


Also the outer prop shaft seal (#21) was improved and still available  OEM #0313730.  The shaft diameter is 1/2".    


Side view of the lower unit/gearbox showing the water pump setting on top and the water intake slots 


Water Intake :  These motors main water supply to the water pump comes from water intakes are on each side of the rear of the gearcase housing, (not shown in the photo above) in front of the prop.  There is an internal screen behind the intake holes.


Free Motor :  The motor being covered here was initially a gift to my neighbor.  This motor had bee sitting for "A Number of Years" and was totally seized up internally.   He needed one for his 10' Columbia fiberglass boat, so maybe this would fill the bill.  He asked me to give him some guidance.  Well first pull the head.  WE only twisted off 3 of the 6 head-bolts in this process.   With the head off, WOW, both cylinders were rusted badly and the pistons stuck/rusted into the cylinders.  He used penetrating oil for over a week and every time he walked by it in his shop, he would rap the top of the highest piston with a hammer on a large wooden dowel, but with no movement at all.  OK, he asked me what next.  Well, we need to remove the block off the exhaust housing and tear it apart to access the connecting rod bolts so the pistons could be driven out the top.



Here is what we saw when the head was pulled,
pretty much indicative of salt corrosion over a long time


First off, we/he needed to remove as much rust from the cylinder walls as possible so that they could be driven out the top.  Clean up as much as possible with a wire brush, then inside the cylinders, using 60 grit emery cloth, try to remove as mush rust as he could, and more penetrating oil.


With the block split, which allowed us to remove the connecting rods, then again using the large wooden dowel, pound on the top of the pistons a bit and then alternating onto the connecting rods that had the caps replaced.  So back and forth numerous times and finally one moved, and we were then able to remove it, then came the other.  Those cylinders were rusted, and from the appearance from a LONG time after being ran in saltwater, then the motor tipped up where water inside the exhaust housing had ran back into the cylinders through the exhaust ports. 


The rings were intact on the pistons, but he broke 2 of the 4 in taking them off.


But until we got to honing them, we were not sure that we could salvage this gem.   They were rusted badly, but not deep and for the amount of usage he would give it, with a lot of honing, in all probability the motor could be salvaged.   Some parts, including gaskets and bearings were not available, but he had already purchased new rings and head gasket for close to $70.   I showed him how to make some of the cover plate and manifold gaskets, by using bulk gasket material covering the part, and a small ball peen hammer tapping around the edges of the metal part.


Here You Are :  Since he had no idea of what was needed, I was his coach, UNTIL he apparently had a chance to really sleep on this project and evaluate the situation.  I then became the new owner of lots of parts, (as seen in the photo below) like I needed another hole in my head.   About the only reason (my sane mind?) even accepted it, would be another project for my outboard motor repair articles here.  No sense to tear apart a perfectly good motor just to get information and dimensions for writing an article.


Here you see the motor, (all in pieces) that I will be covering here


There being numerous parts unavailable, or an exurbanite price, and/or existing ones worn considerably, this project can probably be considered a Red-Neck repair.


The cylinders were honed until MOST of the pitting was removed, going about .010" oversize.


The water pump housing was made of pot metal and had a bit of wear, but not enough to hurt, as I by using sandpaper and a block of wood backing, sanded this roughness out, trying to match the inside contour.


The lower unit's water intake had a homegrown stainless steel screen pop riveted around the outside of the housing.  Upon inspection, there was supposed to be an internal screen wedged inside of these outer intake slots in the lower unit.


The head was warped enough that it had to be sanded down flatter by laying a sheet of 80 grit sandpaper on a flat surface and doing a figure eight with the head on this sandpaper while pressing down on the head.


The one good thing that everything under the flywheel (points, condenser and coils) looked brand new.


The gearbox oil was still good and with no leaks.


The carburetor needed a complete disassemble and clean/soak in carb cleaner. 






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Originated   01-02-2018, Last updated 01-18-2018
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