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

Maintaining

Johnson / Evinrude
 4.5hp / 7.5hp Outboards

1980 - 1983 ,   (Information & Observations)   

The photo above on the left is a 1980 4.5hp with the one on the right being a 1984 6hp model which would be basically the same as the 7.5hp (for comparison, notice the similarities).  

 

 

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.

 

General Observances :  These 4.5 and 7.5hp 2 cylinder 2 cycle motors were made from 1980-1983 which utilized the newer (at that time) exhaust housing/lower unit for that small size motor, and the twist throttle unit used the cable type with a kill button on the throttle end as found on the later models .   Essentially the 7.5hp 1980-1983 motor was utilized as the basis for later 6 and 8hp (1984-1991) motors.  So any information of these 6 and 8hp motor articles would generally apply to those 7.5hp motors.   Both of these utilized  the to later become standard on the 6 and 8hp exhaust housings, transom clamp assembly, shifting and gearbox assembly.  They still used a shear pin at the rear of the prop.  It also had the thermostat moved to the top of the head for the 7.5hp.  The 7.5hp utilized a fuel pump and a separate 6 gallon hose connected fuel tank.

The 4.5hp motors used what appears to be a totally different powerhead but the piston rings are the same as the 7.5hp and the 6-8hp early(1985-86) motors .  These rings were also used on many of the smaller single cylinders motors of that era.  Therefore I am not really sure why both are listed as this family, as the 7.5hp is almost identical to the later 1984-1991  6 and 8hp.

 

And one known 1983 6hp Johnson made in Belgium that was actually a 7.5hp with a restrictor plate between carburetor and the manifold.  So it appears that you may encounter some hybrids in this 1980-1983 era also.

 

The first major difference between these two was the 4.5hp used an integral plastic type fuel tank under the upper cowling basically using gravity feed.   The second main difference in the 4.5hp carburetor has a built-in fuel shut off valve on the outer inlet side of the carburetor, activated by rotating the choke knob.   And the third main difference is the starter.  I can not find a thermostat for the 4.5hp, which may not be bad as most of these small motors are run a a low trolling speed anyway.  One more thing on the 4.5hp, the spark plugs are tucked up under the fuel tank so far, that there is no possible way to access them UNLESS you use a spark plug wrench.

 

Essentially, again even though these two motors are listed as a family, they are indeed individuals because of a different powerhead and fuel system.  As mentioned above, the exhaust housing, lower unit and transom clam are all the same however.  Even the upper motor cowling is the same, with the exception of the fuel tank filler hole on the 4.5hp.

 

Here you see the rear view with the only HP designation

 

In 1984 (at least in the US) they came back out with a 6hp and also upped the 7.5 to an 8hp.  Now somewhere in the mix, I have seen a 1983 8hp, which does not show on any of the lists, so possibly it was a LATE LATE 83, but with the 8hp parts of a 84.  The only difference was that it was painted all white as compared to the gray lower sections of all the known 6hp 84s .

 

Year of Manufacture :  This is important in any repair, for obvious reasons.  Prior to 1979, Johnson used the last 2 digits of the year in the model number.   Here is a LINK to help you determine any of the Johnsons.  And here is the LINK for the Evinrudes.  For those of you readers outside of the United States, here is another LINK that may better help with your motors.

 

After 1979, OMC's manufacturing year code, which included Evinrude, is the word "INTRODUCES", with each letter equating to a number ie:  I=1, N=2, T=3, R=4, etc. as illustrated below.  As before, the last 3 letters are what you are looking for.  Then in this case, disregard the last one, as it pertains only to a model revision that only on specific cases may be reverent to a marine repairman if problems arise.  A code of  J5RCSS would equate to Johnson, 5hp, with no other letter, meaning a 15" shaft, CS = 1980, and the S could be a model revision, so this motor would probably be made in the very last part of the year.  For more detailed information on worldwide production, CLICK HERE

 

DATE CODE

I

N

T

R

O

D

U

C

E

S

YEAR

1

2

3

4

5

6

7

8

9

0

 

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

 

There should be the model and serial number on riveted on 1” X 2” aluminum ID plate on the LH side of the transom mount bracket (looking forward). Sometimes this ID plate can get removed.  If this happens, you can still usually tell which motor it is by looking at the 25 cent size soft plug in the upper rear RH side of the block.  The model and year are stamped in this soft plug also but probably only up to 1979.   You may find that the plug numbers do not match any of the codes, as the factory changed things as time went on.

 

Exploded Parts Lists :  For a internet link to a marine parts seller CLICK HERE  that has exploded views as shown below in this article.    From this business, you can identify the numbered parts ad purchase parts online from them.

 

Flywheel / Electronics :  To get to the electronic ignition which are under the flywheel, you will need to pull the flywheel off the tapered shaft of the crankshaft.  Removal of this nut needs a 11/16" wrench for the 4.5hp and a 3/4" wrench for the 7.5hp.  If it is so tight that you can not loosen it normally, then try to tap the wrench with a medium hammer to jar the nut loose initially.  If that does not work then you will have to resort to a air impact gun, but it should not be tightened this tight.

 

Most times if you just loosen the flywheel nut a few threads, (leaving it about even with the shaft end) then hold onto the flywheel by wrapping your arm around it and lift up enough to put a lot of strain on (almost holding the motor upright) then rap the nut HARD with a HEAVY brass mallet, the flywheel should pop off the crankshaft taper.  The reason for the brass hammer is so it does not damage the nut or shaft threads.  You could get by with a regular hammer if you are careful.  One hard rap is better than a dozen love taps.

 

If this does not pop the flywheel loose, then you will have to resort to a flywheel puller as shown in use on my 6-8hp article.  These usually have 4 slots milled in the top flange to accommodate the bolts to be threaded into corresponding threaded holes in the top of the flywheel.  Here we will be using only 3 that are spaced evenly.  This puller could be also use if only 2 opposing holes were there.  The large bolt in top is threaded into the puller boss.  In use you thread the 3 smaller bolts into the flywheel so that they will allow the puller to set evenly on top.  You then tighten the large bolt into the puller, which puts pressure on the crankshaft end, pulling the flywheel up.  If it comes really tight, then with the large bolt's pressure on the crankshaft, rap the large bolt head with a hammer, which usually will help and pop it off.

 

Electronics on The 4.5hp :  In the photos below you see the flywheel and the electronics system
 

Here you see the bottom side of the 4.5hp flywheel Here is the CDI electronics on the 4.5hp timing plate.

 

Electronics on The 7.5hp :  As these motors used the newer Capacitor Discharge Ignition (CDI) system that uses a power pack as shown in the photo below on the left.  On the RH photo are shown the individual coils and the throttle cable attachment to the timing plate for the 7.5hp.
 

6 / 7.5 & 8hp powerhead viewed from LH side 6 / 7.5 & 8hp powerhead viewed from RH side


Timing plate with electronic coil & pickup sensor
 

 

Spark Plugs :   Spark plugs for these motors need to be 14mm threads with a 1/2" reach.  They REQUIRE this thread length at 1/2".    By this the 1/2" reach is measured from the base of the threads to the end of the electrode and you will probably only have 3/8" of threads.  They will have a 13/16"hex body.   The plugs recommended here will fit the right "reach" length.    If you try to put anything longer in, they will reach too far and possibly hit the top of the piston. 

 

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

You may find a recommended plug of  QL77J4, this is the same plug as above but with a resistor or sometimes called a radio noise suppressor plug.  this one would be recommended of you are running a sonar/depthfinder or VHF radio.

 

Recommended Fuel / Oil Mix :   On these 2 cycle motors you have to mix the oil with the gasoline to lubricate the internal parts of the motor, so you need to know how much gas you will be needing, add the oil before you fill the fuel tank so that the oil mixes better than just dumping it in after you fuel up.  However if that is not possible, then guess, add a majority of what you think it will take, top off the tank and then add the rest matching the amount of gasoline it took to fill the tank.  If you are using portable fuel thanks, before hand, when you fill it, make you a stick gauge, by adding 1 gallon at a time, use a clean 3/8" or 1/2" dowel start filling the tank, mark the dowel at each gallon you add, then you can use it as a measuring stick.  This way you can refill a partly full tank with the known amount of oil.

 

Fuel/oil mix on all the OMC motors that have needle roller bearings (this motor included) is 50-1 of TCW-3 standard outboard oil.  However I have found that since I troll much of the time, other than getting there and back, I like to use a mix of 75-1 but of a SYNTHETIC blend oil instead of the standard oil.  This gives me at least the same or better motor lubrication, plus it gives me less smoke at a troll and the spark plugs last longer before fouling.

 

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

 

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

FUEL MIXING TABLE

Gallons of Gasoline

Gasoline to Oil Ratio

Ounces of Oil to be Added

16:1 24:1 32:1 50:1 75-1 100:1
1 8 5 4 3 2.5 2
2 16 11 8 5 4 3
3 24 16 12 8 6 4
4 32 21 16 11 8.5 6
5 40 27 20 13 10 7
6 48 32 24 16 12 8

 

 

The official OMC fuel/oil mix for most outboard motors in this year ranges has for years been 50:1.  This means 50 parts of regular gasoline to 1 part of outboard motor oil.  Remember that this was before unleaded gasoline or even possibly an octane rating that was listed at the pumps.  Now the recommended fuel is 87 octane gasoline. 

 

Early on for the 2 cycle motors the recommended oil was regular automotive 30 weight motor oil, then the outboard industry came up with any TWC oil, which finally evolved into the latest which is TWC-3 outboard motor oil.  You probably will not see much of the older TWC-2 oil out there anymore.  You can spend your money and purchase OMC oil for about $6.50 a quart, or buy a name brand quart for $3.59.  Sure the manufacturers want you to buy their oil, and for a new motor under warranty, it may be advisable to do so.  But the consensus from many experienced boaters is to use any good brand of oil, as long as it has the TWC-3  rating, it meets or exceeds the manufacturers specifications.  The TWC-3 represents,  Two cycle, Water Cooled, type 3 formula.  The formula 3 has decarbonizing additives designed to be more compatible with the newer non-leaded fuel.

 

So from the above information, you can see that fuel and oils have changed dramatically since these motors were first brought out in 1966.  In this instance, times have changed for the good, as we now have a better gasoline along with way better outboard oils.

 

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

 

Fuel Pump :  As mentioned above the 4.5hp motors use a integral gravity fuel system, BUT it also uses a vacuum assisted system.  The fuel pump (be it what it may) is simply a vacuum line from the RH motor sideplate into the fuel inlet line/shut-off valve.  This is a rather simple unit, being a small pulsating rubber bladder and probably only used when the fuel tank gets empty enough that there is not much gravity to fill the carburetor bowl.  I suspect this may not be efficient enough to pump fuel once the tank has been drained run dry.  More is covered on this below under Carburetors.

The fuel pumps on the 7.5hp are pretty universal of all the other OMC motors made at that same time frame, being the small square one fitting most all these small motors up to 40 hp until about 1987.  Since these motors used only a rope starter, the standard fuel pumps would be the small square version up until the 1987 transition.  For a link to the rebuild procedure for this older small square fuel pump  CLICK HERE

If you suspect a fuel problem, you can check the fuel pump by being able to suck, but not blow into the outlet, blow but not suck out of the inlet.

 

These 7.5hp motors use the single fuel line and the standard OMC quick fuel line couplers.

 

Carburetor :  For those of you unfamiliar with small outboard motor repair, to remove this carburetor, unscrew the three top bolts holding down the top plastic silencer/breather cover (29) in the illustration below.   The "Rich/Lean" idle knob can be pulled straight forward and off it's splined shaft.   Make a mental picture of which way the indicator  pointer is pointing so you can re-install it the same. 

Then unscrew the retainer screw holding on the choke knob (outside front of panel) and remove this knob.   The rear of this plastic choke rod connects to the carburetor choke arm by a small headed pin.  On some motors you may find the pin's head either on the outside of the connector or inside.   Remove this unthreaded pin.  Now you can withdraw the choke rod rearward and through it's hole in the lower plastic body (#28).   From there, the lower breather body can be lifted up and away from the carburetor.

There should also be a foam rubber seal (#26) that may be sticking the breather to the carburetor, if so, a bit of wriggling of the breather body should dislodge it.

The carburetor itself is held onto the intake manifold by two 1/4" SAE nuts, one on each side (#33)  (which uses a 7/16" wrench).

Here the 4.5hp carburetor with the combo choke/fuel shut-off In the center of the cowling is the Slow-Speed (idle)
 knob & combo choke/fuel shut off on left

 

The carburetors used on these motors varied with size of the motor and the year of manufacture.  The early metal top ones (1980-1901) are rather simple units and quite reliable with a fixed main jet, otherwise called the "High Speed Orifice Plug" #24.  The hole size for the 6hp is #35 and for the 8hp is #36.  These numbers relate to drill bit sizes.   These early models have a metal bottom float bowl, while the later ones can have a plastic bowl.

The slow speed idle jet screw (#15 in the illustration below) has many small spline serrations on the outer end that mate with splines in the hard rubber knob.  The placement of this knob on the front cowling makes for only 180 degrees of rotation.  In doing any fine adjustments after final assembly on the motor, if you can not get the knob to rotate as far one way or the other to get a smooth idle, pull the knob straight forward and off the shaft, rotate it 180 degrees so you get a better chance to tune the motor.   Then once I get this "sweet spot", I like to pull it off again and position the pointer on the knob straight down, this gives me s then known return position and about 90 degrees of movement either way if my fuel ratio changes.

The normal number of turns out from lightly bottomed out for the low speed (idle jet) #15 is 1 1/2 turns as a start setting.  Many motosr like 1 1/4 turns out.

The 4.5hp carburetor does not use a silencer.

 

On the RH photo below, the float bowl is plastic and you can see the makings of the "fuel pump",  the bladder is the red part, held in by a plastic threaded nipple to the vacuum line.  The upper nipple is the actual fuel line attachment.

 

The 4.5hp carburetor exploded parts shown below (note the fuel shut-off on the lower LH) Here the 4.5hp carburetor showing the fuel pump


The slow speed idle jet screw (#9 in the illustration above) has many small spline serrations on the outer end that mate with splines in the hard rubber knob.  The placement of this knob on the front cowling makes for only 180 degrees of rotation.  In doing any fine adjustments after final assembly on the motor, if you can not get the knob to rotate as far one way or the other to get a smooth idle, pull the knob straight forward and off the shaft, rotate it 180 degrees so you get a better chance to tune the motor.   Then once I get this "sweet spot", I like to pull it off again and position the pointer on the knob straight down, this then gives me a known return position and about 90 degrees of movement either way if my fuel ratio changes.



The normal number of turns out from lightly bottomed out for the low speed (idle jet) #15 is 1 1/2 turns as a start setting.  Many motors like 1 1/4 turns out.

 

In the illustration above #28 is the fuel pump bladder, with the #31 the fuel shut off.

 



1985  metal top carburetor (same as these 7.5hp) with butterfly choke & metal fuel bowl

 

 

 

Shown below is the carburetor & breather for a 1980 & 1991  6, 7.5 & 8hp

 

Manual Starters :  These motors used two different manual starters, depending on the HP.   The 4.5hp used a unique pivoting gear that when the rope was pulled, the gear unit pivoted up engaging the flywheel ring gear teeth.    

 

Side view of the 4.5hp starter unit.


One unique thing on the 4.5hp is that if the twist grip throttle handle is advanced to FAST, the ratchet disengages and will not allow the motor to be started.

 

Exploded view of the 4.5hp starter unit.

 

The 7.5hp used a unit similar to a automotive type spiral sliding engagement gear, that was carried over from the earlier 6 and 8hp then also into the later 6 and 8hp motors.

 

Exploded view of the 7.5hp starter unit.  Here the 7.5hp starter unit is shown.

 

Cable Type Throttle Linkage :  These 4.5 and 7.5 hp motors used a push/pull cable throttle linkage system.   The routing and attachments are completly different on these two motors.   Adjustment in the throttle positioning is accomplished by unbolting and screwing the outer plastic either in or out if you want to readjust the indicator pointer to the speed letters.

 

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

To disassemble this handle unit, you will notice two small round holes 180 degrees from each other top and on the bottom.  You need to use a couple of punches, depress both of these at the same time, while pulling the handle off front-wise.  A helper can be useful in this instance.  Inside this plastic is a dual rotating cam slot which a cross-pin enters into spacers and rollers which is in turn attached into the cable end.  When disassembling these be careful to not loose this pin. 

 

After much usage the embossed Slow-Start-Fast indicators may get worn, however usually the start position is on top.  The start raised rib on the twist handle is usually aligned with what may have been the Start position on the handle.   When twisting the handle, the cable moves in or out. 

 

The photo below shows the multi -function idle/kill button on the end of the twist knob.  This style handle is activated by a internal cable from the handle to the carburetor.  Inside the outer rotating knob, which is a low speed adjustment, is also a push button kill button.  This rotating button/end knob when used for an low speed adjustment does not have any carburetor adjustment, but is used mostly by trolling fishermen who want a preset low speed other than having to each time they twist the handle to any other position, and want to go back to their preset slow position.  In essence this is just a fine tuned low speed setting.  The knob can be turned in or out.  Out will usually come to a stop after a few turns, where in, once it has passed the few threads on the large inner section of this knob, will just keep rotating, but do nothing because it has simply ran out of threads. 

 

If you can not get it to adjust, the cable to the linkage to the timing plate by unbolting it there and screwing the outer plastic either in or out if you want to readjust the indicator pointer to the speed letters.  This can become frustrating.  However they are a PITA to get really adjusted so most fishermen just forget it and visually adjust the low speed by ear and as seen on the twist grip handle.

 

To disassemble, push in on the 2 plungers simultaneously  (top & bottom) & pull the twist handle forward.  This reveals the slider cams which engage a cross-pin which activates the in/out cable movement. 

 

In the photo below on the right, you will see the black plastic threaded outer cable attachment bolt on the thermostat housing and the pivot bolt at the end of the inner cable/ timing plate arm.  You will also see a spare prop shear pin electrical taped to the plastic outer cable end.



Here the RH side view of the 4.5hp motor showing the throttle cable coming from around & under the powerhead to the side of the carburetor above the powerpack & the fuel tank Here the rear end of the throttle cable is secured on top of the thermostat housing of the 7.5hp motor with the inner end secured at the pivot bolt.  

 
Midsection &
Trim  / Tilt :
 There is a black knob, located close to the transom clamps of engine.  When put in one position, it locks the motor to the cross shaft, effectively locking it in that position, when the knob is turned to the opposite position, it allows the motor to freely tilt up or down.  It has a sketch of a boat with the motor raised on one side of this flat knob indicating it can be raised, but the wording or sketch usually get worn off so that it is hard to tell which way is which and many heavy handed loggers may break off this plastic/Nylon knob.

This lock needs to be in the lock position if you use the reverse gear on the motor so it won't kick up.  However, if you plan on using the motor for trolling in the shallows where you could drag the skeg, leave it unlocked.  Or use the shallow water drive latch located on the starboard side of the lower transom bracket.  This knob is made of black plastic and it it gets gunked up, or sticky AND the operator is not sure which way it needs to be moved, the thin outer part of this knob can become broken off.  At this stage of the game, TS, no factory replacement is available.  And the owner now does not know which way to move it, (which is usually locked into the NON-TILT POSITION).  The only way to get the hosing not tilted or to access the inner part of this knob shaft is to pull the adjustment pin (#23) in the illustration below.  This is the part that the lock fastens onto, so by pulling it out, you can now raise the motor, inspect, lubricate  things, maybe even remove the knob shaft for repair.

There is a fold out carrying handle tucked into the front of the clamping bracket.  The clamp screws are 5/8" diameter and standard National Course threads.  The upper and lower motor mounts are made of rubber and are the same interchangeable part.  They appear to be very durable, plus easy to get to.


Mid-Section  exploded view


Muffler :  These motors have a small plastic cover that covers the upper exhaust vent on the rear of the midsection (#80 in the above illustration).  This does muffle the motor some, but in reality it was called a "water outlet" on these earlier models.  This muffler/outlet allows for some outlet water to mix with the exhaust gasses to cool the housing and at the same time allows only a small amount of exhaust with some spitting water, as an indicator of sorts, some may call it second exhaust port, however in actuality, in these smaller engines you could say it is an idle bypass, because at idle speeds there is not enough exhaust pressure to overcome the under-water pressure out the larger hole near the prop.

 

 So if you see a slight amount of blackish oil coming out of the bottom of this, it is normal.

 

Water Indicator :  These motors have the tell tale water indicator system common with most of the later motors.  Here the 4.5hp is simply a small 5/32" hole on the bottom rear of the block, diverting this indicator water straight rearward and down.  These seem to be a bit more trouble free as related to plugging up as the outlet hose to the overboard tell tale takes off the side-plate cover near the top instead of the bottom.  This helps a bit in that it is less susceptible there to less debris plugging a outer tube.  The 7.5hp would be similar to the 6-8hp article.

 

Water Intake : These motors main water supply to the water pump comes from a screened tube right behind the prop in the exhaust outlet which was carried over from earlier motors.  Water is forced into the screened intake to the water pump by the prop thrust.  Some users report that when running these motors in a barrel, (usually a small one) that the prop has to be installed and usually in gear as apparently they need to get more water pushed to the water pump by the prop to get enough to cool when running  at over an idle.

 

There is no stainless steel intake plate on the left side above the cavitation plate like the previous versions.   Aftermarket flush kits for this motor are obsolete and rather scarce.  There is really no provision currently available for a easy muff system to run this motor out of a tank.  But there is hope.  I think that I may be able, (by tearing mine down) to figure out a way to drill/tap/modify the lower unit near the water pump to where I may be able to utilize the later screw in flush adapters on this motor.  But that is a project in itself.

 

Overheat Problems at Troll or in Tank :   If you happen to have the motor overheating at slow speeds, you might consider looking at the water intake screen tube which is situated in the lower unit directly behind the prop.  This is placed here so that the prop thrust pushes fresh water into this slotted screen tube.   This slotted screen could be inadvertently blocked enough that these holes could partially block water intake at a slow speed.

 

This slotted screen is made of plastic which would be hard to remove and save after many years, plus it is expensive to replace.  So using a small nail or similar tool, you should be able to carefully clean any debris from the holes clear enough to make a difference.

 

When using this motor in a tank, always run it in forward gear as the water pickup tube is directly behind the prop, with the idea the prop will help push water into the pickup tube.  If the tube is partly plugged and the motor is ran mostly in neutral, the motor may not be getting enough cooling water and overheat.

 

Water  Pump/Impeller :  These motors utilize a different shifting system.  To disassemble this for water pump impeller replacement, there is a difference in the shifting rod from any previous motor of this size.  It rotates instead of moving vertically as seen by the #10 & #78 bevel gears (located under the carburetor) shown in the midsection exploded views below .  The upper end of the lower unit shifting rod is machined to match the #78 bevel gear coupler, which should just slide out and down when you drop the lower unit.  The whole shifting rod then drops down and then out the bottom with the lower unit.  This is a lot simpler to disassemble than the previous years.

 

Replacing it back into the midsection housing and re-aligning it after replacing the impeller may prove interesting.  This CAN all happen without having to remove the powerhead, however it may take a bit of jiggling to get things to slide back together.  The first part to align is the water tube, which slides into the long upper snout of the housing.  This snout is considerably longer than most, so it helps get the tube started while you can still see inside the midsection from below using  a flashlight.  The tube's lower end is cut on a angle to help facilitate easy entry into the grommet in the upper end of the snout.

 

Second the driveshaft and finally the shifting rod.  Initially I was worried about the shifting rod's re-alignment, but that was easy.  The hard part was to align the driveshaft.  It took a lot of eyeballing for the lower unit's upper mating surface to be even with the midsection's lower mating surface.  After the driveshaft slid into the splines by slightly rotating the flywheel as you push up, the lower unit slid up to within about 3/8" then it went into the shifting rod gear easily.  You can not see the shifting rod as it goes up inside it's own pocket, but it must have a internal tapered area at the top because things just slide in fine.

 

OMC  / Bombardier #0389576 water pump impeller $18.00    This impeller fits Evinrude and Johnson 4hp, 1980-1992; 4.5hp & 7.5hp, 1980-1983; 5hp, 1996-2005; 6hp, and 8hp from 1984 to 2005 2 stroke engines. 

Water pump impeller #0389576 Water pump kit

 

Now for a confusing part on my 6hp.  Since we are dealing with a used motor with no known repair history, we are not sure what had been previously done.  I purchased the proper identified impeller from the parts manual  #0389576.  Upon removal of the water pump, I found the existing impeller was the right diameter but thinner in vertical dimensions.  The recommended replacement was .845" thick while the existing one in this motor was .700" thick.  As luck would have it the existing one appears to have recently been replaced, so I just reinstalled it.  It was not clear if mine had a replacement water pump OR I got the wrong impeller.

 

The experts say to not reuse a lower water pump plate if it is scored.  Well this one had a couple of deep circular partial grooves in it.  I disregarded the smaller diameter one near where the seal ridge of the impeller rubbed.  But there was one larger wear groove out farther and near the outer edge.  Not wanting to wait for a replacement, I took it to my band-sander and using a worn 240 grit belt, sanded this stainless steel plate from all directions to removed minute metal as evenly as possible.  OH yes, you will need to dip into water after each sanding, as it gets HOT. 

 

Thinning this plate should not have any effect on reliability as long as it was smooth, and this plate uses a gasket both on top and bottom, and the pump housing bolts on top.  Maybe not recommended, but it worked.

 

The stainless steel cup looked in better condition so nothing was needed on it.

 

Lower Unit :  They have a unitized gearcase accessible from the rear instead of the previous gearcase that split horizontally at the prop shaft line.  The prop is driven on by a stainless steel shear pin 3/16"dia. X  1 1/4" long for both motors.  There is a prop shaft nose cap on the rear is not screwed on, but a slip fit that is retained by a cotter pin.

 

The new rotating shifting shaft cam in the lower unit, pushes a internal spring loaded rod that is in contact with the shifting clutch dog.

 

Remember that when you check/change gear oil, it is recommended to replace the drain plug plastic washer (#0311598) as they seem to be the most likely point of a leak.

The water intake is in the exhaust tube right behind the prop as used in the older smaller OMC engines as seen by the screen #24 hanging down in the exploded drawing below.   And it offers no real opportunity to attach a currently made flushing attachment. 

 

One thing they made provisions for was a recessed location in the lower unit to install a zinc anode, #47 in the exploded views below.

 

One thing I may warn you is that when you try to reinstall the lower drain/fill plug, is that IF the plastic washer remains in the hole, you may very easily NOT be able to start the threads straight.  And this threaded hole sets at a slight angle with the outer housing when it is tightened in tight.  If it looks like it is going in crooked, stop before you ruin what threads are left.

 

 If the unit is full of oil, you may be able to lay it down enough to be able to remove the plug without a lot of oil draining out.   A 3/8" X 16 NC tap can be ran in VERY CAREFULLY to clean up the slightly cross-threaded threads.   AGAIN, VERY CAREFULLY.

 

Here is shown the actual gear-case/Lower Unit Here the Lower Unit is in exploded view for from 1980 to 1991 for 6, 7.5
& 8hp with the long & extra long shaft spacers being on the upper left

 

To remove the lower unit on these motors there is one bolt in front that is quite visible (#5 for a short or extra long shaft, or #58 for a long shaft in the illustration above).  There are also two more bolts underneath, above the prop (#56).  Removing all three of these should allow the lower unit to be withdrawn from the exhaust housing and down to where you can remove it to access the water pump housing (#24).

 

If this does not allow the lower unit to drop down, possibly some previous mechanic did not grease the upper driveshaft splines and it is stuck in the crankshaft.  If this is the case, wooden wedges driven equally around the midsection/lower unit may be your alternative.

Propeller
 The prop is a NON THRU THE HUB type.  The prop has a shear pin and the rear plastic bullet nose is just a retainer that is held in place by the cotter-pin.  These props have a rubber hub bonded to the prop along with to the bronze hub.   On my prop the rubber must have gotten some corrosion between the prop and the inner hub, forcing the hub forward about 1/16".  This made it very hard to get the shear pin in place while still having the brass spacer #54 shown above.  I tried to press the hub rearward, but no avail as the rubber was still bonded (or stuck), so I simply chucked the prop in the lathe and removed that amount off the rear of the prop to allow the spacer to be in place and to accept the cotter pin #53.  

 

The prop shaft is .531 or 17/32".  why this odd dia. ?  Well my guess is that since most roller bearings and seals are actually made in metric sizes, this .531 equals 13.5mm, so the seals would be more readily available.  

Prop Specifications ;
  The 4.5hp prop is OMC / Bombardier #0391096, a 8" X 7", 3 blade.   For the 7.5hp, OMC / Bombardier  #390237 — propeller, (Part replaced by OMC 0778797)   8-1/2" X 9" 3 blade, (same for 5/6/8hp models) EXCEPT the long shaft models which use a different one.    Or Michigan Wheel propeller, 1980 & newer Johnson 6 / 8hp #012032 8 1/2" X 9, 3 blade $81.99

Paint :  OMC  / Bombardier  paint, spray can,  Ivory #173744,  

 

 

Copyright ©  2014 - 2017  LeeRoy Wisner  All Rights Reserved

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Originally stated 09-20-2014, Last Updated 04-30-2017

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I have had numerous inquiries about a simple cheap upgrade from the 6 to 8hp motors indicating the removal of a restrictor plate behind the carburetor.   Well in Europe, it was common to downgrade a larger motor, the information I have, this would have been on the 15hp made from 1974 until probably 1992.  However the only motors I have heard of doing this, used a restrictor plate between the carburetor & the manifold was the 15hp, economically dropping it to about 10hp to comply with local HP restrictions.

 

The early smallish motors (other than articles I have written) are the 5.5hp which were made from 1955 to 1964. 

In 1965 the 5.5hp was dropped & replaced by the 6hp which was made up to 1979. 

Then from 1980 to 1983 they dropped the 6hp & brought out the 4.5 & 7.5hp (both on the same pattern) which was made up to 1983.  

In late 1984 they dropped the 7,5hp & went to 6hp AND an 8hp.  This model change (upgrade in a lot of things including the rope starter) was made in 1985 which went until 1991.  Basically this would have been some improvements, but mostly a different carburetor. 

 

In 1992 they brought out a totally different design, lots of plastic & shifter lever on tiller handle, etc. this was made until the end of OMC in about 2005.

 

 

The only articles that I write about are ones that I personally have worked on that motor, so you may see gaps in my articles.  However I have had a lot of requests for information on these older small motors, therefore this is info that I have found on OMC parts lists is as follows.

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.