ohnson, Evinrude, OMC, outboard motor, outboard motor repair, 9.9, 15 hp, year of manufacture, powerhead, piston, water pump, carburetor, long shaft, 15 hp conversion, sailmaster

Note -- some of these articles have pictures that require a possible long load time, especially if you are on a dial up connection

Items Covered Here:  This section will cover all the 2 cycle 9.9 & 15 hp carburetors, carburetor timing & fuel pumps, but keep in mind that there may be a few slight differences depending on the year of manufacture or whether there has been an upgrade made on the motor.

Carburetor :  Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet.   All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end.   It can run OK on the lower end & idle OK because it is using the idle jet then, but it will have less top end power than with the original 9.9 main-jet.   This could possibly also be a problem if you ordered a carburetor repair kit.  The factory only lists one kit for both the 9.9 & the 15hp motors.  If they include both main-jets in this kit, be sure to install the proper one for the 9.9 or for the 15hp.  If in doubt compare the hole sizes from your existing carburetor.

Believe me, as I have been there/done that.  Matter of fact that is what years ago got me into writing the original of this article so I could document & remember myself, as I could not readily find any factual information on the subject, only speculation from so called experts.  To reiterate the above, a modified 9.9 carburetor using a 15 hp main jet WILL NOT work, (BEEN THERE, DONE THAT) it will run, but less power than it was with the 9.9 main-jet as it will have too much fuel for the air ration & be blubbering all the time.

Factory specification rating for RPM of  the 9.9hp is at 5000 & the 15hp at 5500.  With operating range for the 9.9hp at 4500 to 5500 RPM, & the 15hp at 5500 to 6500 RPM.

Also there is a neoprene seal on the idle jet shaft that needs to be there otherwise there will possibly be air pulled in around the threads.

Carburetor, early :  Early production motors had all metal carburetors. Then from about 1980 to late 1987 the same basic carburetor as previously was used, but had a black plastic bottom fuel bowl.  This plastic seemed to be an improvement in that if water had entered in the system & got in the bowel, then sat for a while creating RUST inside the carburetor & even enough rust on the outside if used near saltwater to corrode holes in it.  If you have to replace the metal bowel the cost is $24.00 as of 2005.   I have however seen metal fuel bowls on 1982 to 1987 carburetors, however I am not sure if the carburetor was ever changed or just the bowl.  Or there may have been a supply of metal bowls on hand on the dealer's shelves if the plastics had gotten cracked.

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

If the float bowl gasket on the carburetor is black, it is the older composition cork material used for bowl gaskets, & the float will usually be cork also, then you are probably way overdue for a overhaul.  There could be pieces of this cork gasket floating around inside the bowl, occasionally plugging the main jet.  The float should be OK unless the varnish has deteriorated.

 On the early carburetors, the little float needle valve clips did not exist. They have become necessary because modern soft tip needles can stick shut. The clip uses the weight of the float to pull the needle valve open.   If this clip is not in position you are likely to suffer from fuel starvation, rather than flooding.

In the LH bottom photo, you will note that the cam roller pivot arm is white plastic & with an adjustment screw.  This would have been a later unit in that series because the early arms were made of steel with no adjustment.  Some of these carburetors are missing the throttle cam roller & arm unit.

	Carburetor, 1974 to 1979 metal fuel bowl	Carburetor, 1980 to early 1987 plastic fuel bowl	Late 1987 to 1992 with plastic top & fuel bowl
RH VIEW
LH VIEW

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

One thing to be careful is when reassembling these plastic topped carburetors, DO NOT over tighten the cover, as you will break it.

They were apparently designed for easier manufacture as the plastic top has the idle jet in it & the juncture between this top & the main body serves as a passage (depending on the the gasket) for the fuel from one chamber to the other.  There is a white nylon collection box that is sandwiched & gasketed between the main body & the bottom fuel bowl that has a removable main-jet on one side.  These carburetors appear the same for the 9.9 & the 15 hp with the exception of the main-jet AND the throat dia. in the main body.  In the parts list, the main-jet for the 9.9 is spec'ed out at a hole #34 (.055dia), while the 15 is #54 (.110  dia.).  The idle jet suction tube comes off this collection box & goes up into the plastic top for suction to the idle jet itself.  This arrangement appears to help keep debris out of the jets.  

The choke butterfly is the same as the previous models, but the throttle plate is smaller.  The internal throat of the 9.9 is about .500 dia. while the outlet hole into the manifold is .750 dia.   This is quite a bit smaller than the earlier versions, & the guess is that this newer carburetor is probably more efficient.  It appears to have all the screw holes & outboard fittings in the same locations, but whether it would interchange with the earlier versions & function reliably is something I have not tried, but I suspect it would function OK.  However some of the the internal castings of the intake manifold appear different also. OMC repair kit  #439073, or NAPA /SIERRA part# 18-7219.

If it is more efficient,  then the motor will possibly idle better & have less of a black oily residue ooze out of the lower part of the exhaust housing for weeks after it is put into storage.  One other observation while the carburetor is off, you will notice the intake manifold has a slight vertical separator on the bottom section up about 3/16".  The supposition is that this may help fuel flow to the individual cylinders better.   These later carburetors also had an adjustment screw for the cam follower.  

Both these older & newer style carburetors up to 1992 use the same air box or silencer, but to most of us it is known as the air breather.

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

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

Update Carburetor Top Cover:  There seems to be a lot of unanswered questions on this subject.   With the help of a couple of readers of this article, I have been able to come up with the following information.  Mike Hull from Texas supplied enough info to put together the following. 

The new style black plastic topped carburetors used on motors made from late 1987 to 1992 had a habit of the plastic top warping or cracking.  The crack begins near the front hole for the idle needle jet & running along the top of the cover over the needle cavity.  Warping also occurs at the middle rear.  Either could allow air into the idle circuit making for a rough running motor.  This could also contribute to a hard cold starting motor. 

The factory came up with a reinforced top cover to rectify this situation.  The fuel circuit on the new cover is different from the older cover & the gasket that fits between the cover and body is also different.  This new cover used the same number of screws as the old cover did. 

From what I can find out the older covers can be identified by the pivot for the throttle cam roller with this pivot being a metal screw that screws into the top cover.  The later update covers utilize the pivot post as integral with the plastic top & use a retainer washer held in by a O-Ring for the roller arm as shown below. 

The part numbers in the replacement parts & prices as of 2007 are as follows:

Carburetor body cover #336049 (superseded to 433973)   Cover gasket #334043   Low speed longer needle  #432760   You would also need the cam follower washer  retainer O-Ring  #321117.    Or they would be included in carburetor repair kit #433973   ($34.00 as of 2007)

The repair kit lists as being for, 10/14 hp  87-91, 10/15 hp Commercial models – 92.   And 9.9/15 hp recreational models 87-92 using carburetor with separate black cover (original part # 331735 (superseded to 433973) .

Carburetor Repair Parts Found : One of the readers, Bill Smith who operates Claremore Outboard Service, tried to find a solution to this problem for a 15hp.   "I never found a kit listed for the top cover.  I just started comparing top cover part numbers starting  from 1987 and worked my way up to the (1991) motor's parts list.  That's when I found that the top cover had changed to #435356.  Since the top cover gaskets for 1987, thru 1992 had the same part number I assumed (correctly) that the "late production" top covers for the 1991 and 1992 would work on my 1989."  He also found a change of a small groove added in the low speed needle #432760, which is now the same for both the 9.9 & 15hp. 

The cam follower screw is done away with & the later cover has it's own integral plastic pivot peg.  You use the old follower washer but it is held onto the new peg by an O-Ring #321117.   I would also change the gasket #334043 at this time, same gasket as the older unit, but the small sealing ridges in the new top are slightly different in places & the old gasket may not seal completely.
 

Late carburetor low speed needle on LH side showing a slightly modified needle	Late carburetor cover #435356 shown on LH side, note the cam follower uses integral plastic post instead of metal screw

Carburetor Repair Kit Found : The kit shown in the photo below appeared on e-Bay.  This appears to be the missing repair kit to solve the above problem.  The seller appears to have spent some time going thru the BRP Parts lists & found that kit #436342 appears to fit many models 5hp thru 20hp from late 1986-1992.  It also includes a air box (breather) gasket & a new low speed needle (in small bag).  However this kit does not show in any parts catalog listing, so it must have been in a update listing to the dealers so most of the newer mechanics or independent dealers don't know about it.   This kit is still available at a price of about $35.00.

One thing to note is the slow idle speed jet is no longer at the front of the carburetor.  You close that one off and the adjustment screw is on the side so you have to adjust it with the upper cowling off.

 

OMC carburetor cover kit #436342

Original configuration  89 carburetor	Altered  89 carburetor

The 1993 and newer carburetors again changed the design using 6 cover screws instead of the 5 used on the 87-92 carburetors.

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

You also need to remove the recoil starter unit. The manual starter unit can be unbolted with the one 9/16” headed bolt head in the center of the recoil unit.  Be very careful as that spring is dangerous! Pull the rope out a few feet and tie a slip knot in it so that it can't rewind back.  Carefully remove the starter handle, then grab the rope inside at the starter pulley, then let the rope withdraw somewhat but not completely (leave a foot or so to work with).  Now, secure the rope inside at the starter pulley to the upright pulley gear guide in a manner that will prevent the spring from rewinding the cord etc.  Hold the manual starter assembly together when loosening the top large retaining bolt (do not remove that bolt completely from the starter housing assembly).  Carefully remove the starter assembly while holding it together.  Screw a 3/8" nut on the bottom of that large retaining bolt to hold the starter together while you're working on other components.   

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

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

In the illustration below is the dimensions for it if you need to fashion one.  These appear to all be the same from 1974 up until about 1986 & after that there is a different number that has a slight "V" bend front to back.

OMC # 312830  Link, cam follower

Clean & Rebuild the Carburetor :  Remove the fuel line.  Disassemble the unit.  Once the carburetor is off, unscrew the bottom screws that hold the sheet-metal bottom pan on.  When removing this pan, be careful & try to save the gasket if you are not going to get a repair kit.  Remove the cross pin that holds the float, remove the float & the needle valve.  You will need to also remove the idle shaft screw, count the revolutions it takes before it comes out.  In this way you can replace it with the same number of revolutions when you reassemble it.  If the insides are dirty, you can get a can of carburetor cleaner & spray on or soak until things soften up.  Usually you can use an air hose to blow thru the passages & be sure they are clear.

The illustration below is a copy of OMC carburetor repair kit instructions dealing with the 1974-early 1987 carburetors.

In the illustration above, you will notice the #6 needle valve & seat that the needle has a small wire clip shown.  This was not on the early, (possibly 3 or 4 years of production).  Where this clip goes is, it slides into a groove in the needle & then the other end goes over the flat part of the float pivot plate just ahead of the pivot pin.   In functioning, it ensures that the needle is retracted when the float drops as the fuel is used.  It could be possible that if the needle were to stick up (closed), & did not open to allow fuel back into the carburetor, the motor would starve for fuel. 

Part #10 is listed as a low speed needle bushing & was only used from 1974 up to 1978.

There is one solution that works great to soak carburetors in, it is any of the outboard de-carbonizing liquids.  One that is universally available at even automotive supply stores is Sea Foam  http://www.seafoamsales.com/ that is used as a fuel additive to help unstick automotive hydraulic lifters or run it in the gas to decarbonize piston rings.  It will not damage any of the rubber parts.  Just soak it overnight.  Another solution is Pine Sol household cleaner.

It is best to remove the core plug on top of the carburetor to access & clean the well area and 3 tiny holes leading from the well to the rear of the carburetor throat.  To remove this plug, gently drill a small hole through it, (made of thin aluminum) being very careful not to hit bottom when you go through.  Insert a ice pick or bent nail & pry the plug out.  Exposing the idle circuit mentioned!  You need to do so as these are the primary passage ways for the starting circuit when the choke plate is closed and the idle passage ways when at idle.   

 Clean then replace the plug.  A replacement plug is part of the carburetor repair kit.  To install, locate the new plug, get a punch of just slightly smaller diameter than the plug and flatten out the new plug to seal it in place; easy, but be careful not to whack it so hard you crack something.   If you do not have access to a kit, you can reshape the plug & reinstall it, then cover the hole with JB Weld.

However just soaking it in cleaner is not cleaning it.  The jets need to be removed & checked for no debris, The small passages leading behind both the main jet & idle jet need to be free of debris.  Usually you can blow out these passages with compressed air.  But if you are unsure, read on.

You can possibly get by without a repair kit or gaskets if you are careful in your disassembly do not use a gasket sealer when you reassemble it as some may dry out when the motor is stored over the winter & some of this sealer may dry out & chunks of it can get in the main-jet.  Later gaskets were a neoprene type positively does not need any sealer.  If it just had debris & water in the bottom, you can get by without a repair kit.  But if it is gummed up varnish you will need to go farther.  Check the idle needle valve for straightness of the tip, if it got dropped, this tip can get bent.  If you are good, you can possibly straighten a bent one, otherwise just get a repair kit.

For the price of a $12.75 repair kit for the early versions, while about $21.75 for the later style.   You get all new gaskets, needle valve, main jet & new metal plugs that are needed to replace the old ones if you have to get all the channels cleaned out, and float adjustment instructions.

OMC part number 398453 Carburetor kit for 1974 thru 1986	OMC part number 439073 Carburetor kit for 1987 thru 2006

When reassembling, place the float back in position & replace the cross pin.  Be sure that the small wire clip is attached to the needle & the tail is over the float plate.  Tip the unit upside down & check the float height.  The now upside down unit should have the top of the float parallel with the now top of the housing.  If not, you can adjust it by bending the metal tab that acts as a stop for the float.  Be sure that the float & needle can move up & down freely, otherwise if either binds, the motor may run at different speeds depending on how much your boat is tipped to one side or the other or raised in the shallow water drive mode.

Screw the carburetor's bottom pan back together & reinstall the carburetor onto the motor's intake manifold.  Remember to reattach the small spring on the starter ratchet if it was made prior to about 1982.  On the later plastic topped carburetors, you will find numbers near the attaching screw locations.  These are for the tightening sequence.

When reassembling the idle jet needle screw, & you forgot how many turns it was set at, a good starting spot would be to screw it in until it lightly bottoms out, then back it out about 1 1/2 turns for the pre 87 carburetors.  It has been found that for the post 87s, that have the plastic top, the presetting is about 4 turns out, probably because of the finer threads on the needle as compared to the earlier needles.

The repair kits shown above on the right #439073 and #431897 appear to not include the main jets.  They also cover a variety of motors, that is why 3 body gaskets are included, you will need to identify the proper gasket by comparing to your old gasket as these kits fit  6 & 8 hp 2 stroke 1986-1996,  9.9 & 15 hp 1989-1996 2 stroke, and 20 hp 1991-1996.  This does not mean a 6hp carburetor is the same as the 20 hp, but just that the repair kit parts fit all those identified.

Reinstall & Adjust the Carburetor :  After reinstalling the carburetor to the intake housing & you need to reinstalled the choke knob.  The choke knob shaft has a slot in the middle that the carburetor's choke lever goes into.  This shaft needs to be installed with the longest length of the slot upwards, otherwise the choke lever does not have enough room to function. 

Now you need to re-install the plastic carburetor intake cover (air breather).  

The idle knob is next, it is the 25 cent size plastic knob that slides over the serrated idle jet needle shaft, & & is about center of the front lower cowling.   The cowling under this knob says "Slow Speed Adjustment", which is misleading as it is really a idle adjustment. 

I try to set this knob with the pointer protrusion down so I have about a 1/2 a turn either way.  As it will not rotate totally around because of a stop bump on the front cowling.  Even after you replace this idle needle jet in the same location as it was before tearing it down (same number of turns out from lightly bottomed out), & you get it running again, you may need to fine-tune adjust this idle knob for easy starting & trolling.

Front panel, showing choke, carburetor idle adjustment (Rich/Lean)

When you get the motor warmed up & running in gear at near your trolling speed, then turn the carburetor idle screw knob (lean/rich) one way or the other until the motor changes & either runs worse or better.   By rotating this knob a slight amount, let the motor run for a few minutes, & rotate it slightly more in the same direction again.  Doing this you are making the motor run richer or leaner at an idle (mixing more or less air into the idle mixture).  Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire).  At that point, back out the valve 1/4 turn.  Within that 1/4 turn, you'll find the smoothest slow speed setting & the smoothest the motor will run at slow speed. 

From this point the idle must be adjusted again richer to provide the additional fuel necessary to accelerate crisply while under load.  Test acceleration, any hesitation, lean miss, or cough, make slight adjustment richer on the idle and retest.   Repeat to perfection.   The end result may be as far as 1/4 turn richer than 'best' idle in Neutral.

If you get to where you can not rotate it anymore, but need to turn it farther, pull this knob off frontwards, rotate it 180 degrees, push it back on & now try to twist it some more.  The front cowling has a bump on the top of the knob hole, stopping it from being twisted in more than needed.  But since this shaft is serrated, you can slide the knob on in any position.

When you have finished the above adjustment, pull the knob off forward & reinstall it so the pointer in the "DOWN" position. You will now have a "GO TO" position if need be, which will be discussed later.

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

Adjust the Slow Speed Stop Pre 87 Motors: There is also a large slow speed stop knob on the left hand side of the lower cowling on all of the pre 87 motors.  This large knob activates an internal threaded rod that acts as a stop for the throttle plate linkage & is used for a slow speed setting without having to be sure that the twist grip is in the same position each time you want to idle or troll, (not to be confused with the actual carburetor idle settings).    This knob is rotated from the outside of the lower cowling to make contact, stopping the throttle linkage at a desired location.   This shaft is shown contacting the throttle linkage at the red arrow  in the photo below.

Slow speed stop shaft limits movement of throttle linkage to timing plate.

After the motor has been run enough to have it warmed up, try to let it idle.  If it is running so slow that it dies this adjustment may be not stopping the throttle linkage arm to bump the linkage tab so you can adjust this  knob stop screw on the motors left hand side.  It may be best to initially do this with the motor cover off, so that you get an idea of which way to turn this screw.  Turning this knob counter-clockwise pulls the threaded rod out & makes the motor run slower.  Again this is not the carburetor idle jet, but a slow speed stop adjustment.

Adjust the Slow Speed Stop Post 87 Motors:  After 1987 the side stop knob was discontinued when the cable throttle system was instituted.  The new slow speed setting was then changed to a twist knob on the end of the twist throttle.

This info is coming when I get around to it.

Type 3,  cable type, red kill button on end, but with man overboard switch there also	Type 4,  cable type, red kill button & with man overboard switch farther back on side

Carburetor Timing :  The throttle cam plate that rotates under the flywheel when you twist the twist grip & the plate has an raised line type mark on the front of it.  This mark needs to be timed so that when you advance the throttle twist grip, that the carburetor roller is at this mark when the carburetor throttle shaft just STARTS to turn open (RED ARROW) in the photo below.  To adjust this on the older motors, you loosen the 2 bolts on the RH side of the plate & adjust the cam plate to where it just aligns with the roller at the same exact time that the throttle plate begins to rotate. For the newer motors, the plate is shown below is made of plastic & has a adjustment screw on the outside (PURPLE ARROW).  They also have another adjustment screw on the throttle cam roller unit.

To tell when the throttle shaft starts to rotate, take a small alligator clamp & attach a short wire to it, as if you were attaching a electrical wire under the screw head.  Clamp this alligator clamp indicator unit onto the end of the throttle shaft (BLUE ARROW).  Now you have some indicator way better than just watching for rotation of the shaft.   The flywheel & air box were removed here for clarity. 

In the photo below, you can see the timing mark by it being the lighter colored raised part at the right of the arrow's point.  The older aluminum timing plates also have this same style of mark.  This timing plate needs to be adjusted as the roller is away from the timing cam plate by probably .030" at an idle & just touching when the carburetor throttle shaft starts to move.

One thing that needs to be pointed out is that if you change carburetors & or complete ignition systems is that possibly the two may not really be compatible.  Meaning the carburetor roller can be a different diameter.  If you happen to get one that the roller is smaller than the original, you will get a situation like the photo below where the roller does not touch the cam plate mark when it should.  This would give you an engine that may starve for fuel & hesitate before you advance the twist throttle more.  And it may never achieve full power.

The timing mark on the timing plate cam does not need to touch the roller at the low end (or idle) of the twist grip rotation.  On these motors, at low speed, the the timing plate will move counterclockwise AFTER the carburetor cam movement& throttle butterfly valve will stop.  This allows the timing to go where the motor runs at it's slowest before it would die, while not moving the butterfly valve, & if the low speed side of the carburetor, (the carburetor air jet idle screw) is set right.

Alligator clamp indicator	Timing the carburetor throttle shaft to the ignition timing plate

One reader had problems setting the idle adjustment.  "I could set it up just perfect, then after a few revs of the throttle open and closed, the idle would be messed up again!  Either too high or two low.  After disassembly, I noticed a worn spot in the throttle cam roller, since this directly translates to tiny movement in the carb butterfly at idle, it makes a huge difference to the idle speed depending on where this wheel stops turning when at lowest throttle."

His situation, I suspect was that the timing between the roller & timing plate was set slightly retarded as this roller should not have been touching the plate cam at an idle.

Carburetor cam roller worm

If you are having problems locating a start position on your twist grip throttle setting because of obliterated wording, or if you have a remote throttle, a method of locating the timing "START" position would be to advance the timing plate to where the carburetor arm roller just touches the timing plate cam, then advance it about 3/8" (9-10mm) more.  This positions the timing & throttle at about the start position in the carburetor.

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

Second to check would be to whether fuel is coming TO the motor.  Is the fuel line connected to a VENTED tank.   It works best if you hold the primer bulb upright, with the outlet upward when pumping it.  If the check valve spring is weak, this will help the pumping by not allowing the fuel to bypass & go back into the tank.  The primer bulb should become hard after about 3 or 4 pumps when the carburetor bowl becomes full & the float valve stops the fuel from bypassing & being forced up into the carburetor flooding the motor.    After the motor has run enough to remove this pressure of the internal fuel you have pumped, the bulb should become soft as the fuel flows thru it.  A simple way to check if it he tank, pump bulb & line is functioning is to use a nail & push in the small metal ball on the motor end of the fuel line coupler.   While you are holding this ball in, pump the primer bulb.  If fuel gushes out this coupler & all over your hand, everything is OK up to there.  

A faulty fuel pump would not stop it from starting if you pumped the fuel line primer bulb up.  If everything else is OK & the carburetor bowl is filled by pumping primer bulb, the motor will start, it then won't stay running (over a couple of minutes) once the fuel in the bowl is used up if the fuel pump is bad.

If the motor still fails to start & the suspect could be fuel, I would then pull the fuel line from the carburetor, & crank the motor over.  Fuel should spurt out of this line.

If with the motor running, you happen to have fuel spraying out the small hole on the back of the fuel pump, you have a ruptured fuel pump diaphragm.  That hole is a vent hole behind the diaphragm which allows the diaphragm to pulsate, sucking the fuel from the tank.  If the diaphragm itself has a hole in it, fuel is passing through that hole from the other side.  It also provides a visual indication of a fuel pump issue.  Under circumstances like this, your motor may run at lower speeds but appear to starve at higher RPMs.   In essence it actually may have to much fuel because of the pin hole in the diaphragm, pumping fuel directly into the crankcase & since this motor is a 2 cycle which uses the suction of the bottom side of the pistons to suck the fuel in & then forces it out the intake ports of the cylinders.

One symptom of the above fuel pump leak is that the top spark plug may be always richer, (plug wetter) than the lower one because the fuel pump impulse hole on the block takes it's pressure/vacuum off the top cylinder & as the crankcase has the cylinder chambers separated, any extra fuel will stay in that cylinder system.

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

The older pumps are basically the same as the new ones with the exception that the new bodies have 2 external tabs, that align with the diaphragm tabs & other gaskets to help make it easier to assemble the parts correctly.   The repair kits do not even list a kit that fits the same basic fuel pump before 1982.  You however can use the new kits in the older pumps if you are careful & replace the new parts just as the older parts came out.  Price on a rebuild kit is about $12.00. 

There are 2 spring activated check valves, one on the suction & the other on the outlet.  There is also a small spring & plunger inside all this, making it slightly hard to keep everything in the proper locations while making the final assembly.  It is suggested that you use the 2 longer attaching screws as guides in from the back side to align things before the final assembly.  Do not be surprised that you may have to do back & reassemble it a time or two before you get it right.  One word of caution, do your repairs on a clean LARGE uncluttered table as the parts are small &  may tend to crawl off & hide.  Also read & understand the instructions that come with the kit.  CLICK HERE  for link to 1959-1987 factory rebuild info.  Or HERE for 1987 -1998

Early fuel pump from about 1959 to 1987	Later fuel pump from about 1993 on

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

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Copyright © 2004 - 2012 LeeRoy Wisner  All Rights Reserved

Originally stated 06-1998, Last Updated 02-01-2012
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