Outboard Motor Carburetor  Repairs




This Page Under Construction with notes compiled from many articles
Just need to sort them out & add to it



If you frequent the outboard motor repair forums, the assumedly cure all for motors that do not run right if they have been setting for a year or so, is to tear it apart, clean/reassemble th


e carburetor.  This may not be the cure all, but it sure does not hurt to be sure the carburetor is free of rust and debris.  Normally on small single carburetor motors, if you are going to let the motor set for some time, you should run the motor enough after you disconnect the fuel line from the motor to where it uses all the fuel in the carburetor and dies.  This then pretty well assures that the picture shown below does not happen, or if it does the rust will be minimal. 


The one thing you do not know when you purchase gasoline, is just how free of water is it?   Most all of the larger boats use a water separator filter system.  But unless you live in an area where the fuel is known to be contaminated, these separators are hardly ever used in conjunction with small motors of the size we are covering here.


If there is any water in the bowl, it will usually block the main-jet and stop the motor from starting or at least make it run erratically.  Rust will also do the same thing by blocking or partially blocking the jets.   Now if there is water in the fuel of the carburetor that has not been drained, the result can look like the picture below.  You will note that on the newer motors, OMC did away with the metal bottom and replaced it with a form of plastic/nylon, which would eliminate this rust kind of problem.  However you could still have the water or a dried gunk problem.


Rust in this old metal carburetor bowl, not a good thing, it is understandable why they went to plastic bowls.  


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. 

The carburetor shown below is the basic simple standard unit but from about 1971 on using the fixed main-jet style.  Earlier motors prior to about 1970 had an adjustable main-jet in place of the #29 plug screw shown in the illustration below.  You will have to loosen and remove the rewind starter spool so you can get to the throttle lever pivot nut so you can then get a wrench on the port side carburetor nut.


Exploded view of carburetor Here is a close-up top view of the carburetor, note the pea sized welch plug on top & the 2 lead shot plugs driven in to seal holes on the side


You can usually tear the carburetor apart, soak it in a carburetor cleaner overnight, blow it out with compressed air and reassemble without purchasing a repair kit.  Just spraying carburetor cleaner in it does not constitute cleaning even if stripped down.  You may luck out and not have to remove the pea sized aluminum welch plug on to rear center or the 2 lead shot sealed passageway covers.  If not, then plan B is do it over but remove these plugs and clean everything including the passageways under the se plugs. 


The float is varnished and may look cruddy, but if it is still intact and seems to float in gasoline it should function.  All that is usually needed is to check to see that the float level is with the bowel flange when assembled without the bottom bowl on and turned upside down.  If not parallel, then bend the metal stop tab on the float needle valve area to adjust the float level.  If the float is not set at the correct level for the shut off point under pressure from the fuel pump in the carburetor, this could cause motor to run lean or rich. 


Some carburetor repair kits cover a wide range of HP ranging from 5hp even up to 20 hp, however do not think just because the kit is the same for a 20hp as it is for the 6hp that they use the same carburetor.  The kits may have numerous extra parts, as main jets where you just use the one required for your motor.  The breather throat of the carburetor may also be a larger diameter for different hp of motors.


The main jet orifice in the bottom of the bowl has a recess on these models and be full of crud, even though they look clear.  On the earlier models (pre-70) there is a adjustable main (high speed) needle jet on the front lower bottom, this needs to be removed for cleaning.  On the later carburetors there is a clean out plug in front that you can access this orifice.   Use a proper fitting screwdriver to remove the orifice, but be sure there is no debris in or behind it.  Be sure the plug screw has a good sealing gasket.  This plug is in the same location that the early carburetors had a adjustable main needle jet.


Inspect the idle needle jet to see if it is bent or has the tip broken off, (on these the tip does not come to a sharp point) straighten if need be or purchase a new one. Reinstall this jet needle / rich-lean screw, check the packing nut for tightness to insure there is no air leak around the shaft.   The correct way to adjust this low speed needle, is to turn it all the way in until it is lightly seated.  You then turn it back out 1  1/2 turns.  That becomes a reference point for further adjustment.   I like to be able to get a equal rotation each way so set the knob at about what would be 4 on the top.  This should give you a basic setting.  Once you get it where you want, you can pull it off , then reposition the knob where you want it.


With the motor warmed up and the twist throttle in a SLOW position approximating your intended slow/trolling speed, turn this low speed needle jet in about 1/8 of a turn, let the engine run for about 15 seconds to respond to your new setting.  Do it again then wait for the results.  A cough is indicative of a rich mixture. Too lean it will just die.   When you make this change and the engines either sputters or wants to die, back it off the the last position then this should be close to the ideal setting.   If you go to rich it will cough AND then die.  You can now pull the idle knob forward and off the splined shaft, reposition it so that the knob pointer is down so you will have movement either way from this position for fine tuning if needed.  This low speed setting does not effect the high speed running of the motor.  However it you set it too lean, you may later have a issue with it not wanting to start when it is warm.


One thing if you look at the top of this carburetor, the 1/8" hole at mid-section topish is not plugged with a lead shot like all the rest.  My thinking was did I loosen it and loose the plug?  Well, after I got it running I covered that hole with my finger and the motor wanted to die.  After numerous tries, I decided it was not supposed to be plugged anyway and probably there as a vent to alleviate any vacuum inside the float bowl.


In reinstalling the carburetor to the intake manifold, you may have to position it so that it just starts onto the studs, then start threading the nuts on equally before finally tightening them.  What I am saying that there may no be enough room to start the LH nut if you have the RH nut tight.


There is no breather box on this motor as the mouth of the carburetor is open behind the front of the cowling.





In the photos below you can see the bottom or high speed jet along with it's linkage to the control knob. The knob operating this jet slides over a octagon sleeve around the idle jet shaft.  The low speed needle which is the top one, screw it in to a lightly seated position and back out 1 1/2 turns.  The high speed needle which is the bottom one is 3/4 turn out from lightly seated position.   Run it in gear under load (at a fast run), adjust high speed in until it starts to die then back out until it runs smooth. Slow down to idle and adjust low speed in until it slows down or starts to cough and back off until it smoothes out.  You can loosen the high speed linkage rod and adjust the linkage so that the dial is set where you want it.  I also like to slip the low sped knob onto the shaft so the pointer is pointing straight up.  That's all there is to it.  


If you are having starting problems, and you are sure the ignition is OK, fuel or the choke may not be operating properly for cold start.  Spray come mixed fuel into the carburetor and see if she fires off.

If you are having problems with the high and low speed needle shafts being loose and backing out while running changing the adjustment, there is a "packing nut" around the shafts at the point where they enter the carburetor (as seen in the photos below).  Tighten these nuts to put more tension on the shafts, (righty tighty-lefty loosy).

Anyway, it shouldn't take more than 10 minutes to pull the intake manifold and check the reeds to see if a bug or whatever is blocking them open.  But they're rarely the problem and the problem can be pointed to if it won't crank at all without fuel being sprayed directly into the cylinders via the plug holes.

Right side view of a 59 Left side view of a 59



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).

The carburetors used on these motors varied with the year of manufacture.  The early metal top ones (1984-1985 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.  My motor likes 1 1/4 turns out.


Shown below is the carburetor & breather for a 1984 & 1985


1985  metal top carburetor with butterfly choke & metal fuel bowl


The photo below is a OMC 9.9/15hp carburetor from 1974 to early 1987 showing the removable plugs



The later carburetors used a plastic top.  The early with these plastic tops still utilized the front mounted idle mixture screw.  Then about 1988 or so this screw hole was plugged and a rear starboard side adjustment screw was used.  These plastic topped ones used a internal box like collector that the main-jet was screwed into.  This system appears to help by separating to some degree any small debris from being sucked into the main-jet.  

The late intermediate style plastic top carburetor with side idle adjustment.  These also used a plastic bottom fuel bowl. 

Note in the photo below that the idle adjustment screw is moved to the rear side and the front hole is plugged plus that it has no choke as it uses the fuel primer system as indicated by the lower fuel line fitting which is the inlet for a choke/fuel primer as mentioned below.


Here is a 1989 8hp carburetor

To remove the carburetor from these motors, you will need to remove the breather box cover (3 screws)  then 2 more screws that hold the bottom of the box to the top of the carburetor.  Also the choke rod needs to be removed.  This is accomplished by first removing the small Neoprene O-Ring that acts as a retainer on the outer end of the choke retainer pin.  Pull the pin and then push the rod in and remove it from the rear of the front panel.


Also the starter spool will have to be partially removed.  Remove the 2 upper bolts, and carefully lift the spool out of it's base.  Tip it forward to give clearance for the 7/16" open end wrench to access the RH facing rear carburetor retainer nut.   Remove the carb cam roller pivot screw.  When both nuts are removed, you can pull the carburetor forward and then remove the inlet fuel line.


You can then remove the  bottom bowl, the float pivot shaft and lift off the float assembly which includes the inlet needle.  Place all parts so you can easily identify them for reassembly.  You can then unscrew the brass main jet and or idle jets.  If you have the older metal topped carburetor, it may be best not to even try to remove the slotted inner brass idle tube.


On my 1984 6hp the inner (rear) carburetor throat diameter is .810"  with the mating hole in the intake manifold being only .612".  This hole in the manifold is not cast but machined not true with the carburetor's mating hole, but slightly lower by about 1/8" or the top of the hole even with the top of the carb mounting studs.  Possibly one difference in the 8hp is this manifold diameter is larger.

In re-assembling any carburetor, IF it has a metal tag under one of the bowl screws, remember to put this back as the numbers on this tag may be critical if you ever need a repair kit.


Air breather or air box as they are usually called by the manufacturer on this motor is rather unique in that this motor appears to be somewhat semi-super charged.  By this, I mean the actual breather intake is mounted very close to the flywheel ring gear starter teeth.  In operation the rotation of the flywheel forces air from these teeth into the carburetor air box.


On these plastic topped carburetors, there is a slow speed stop adjustment screw (#31 in the illustration above) on the port side about the middle.  This just screw just adjusts the throttle shaft butterfly as a low speed setting.  It does not fine tune the idle jet (#15 in the illustration above) which is either on the center front of the plastic top or on the rear starboard side (depending on the year of manufacture).  You may have to adjust each individually, but in synchronization to get a good low trolling speed.


Choke / Fuel Primer :  These motors used 2 types of choking systems.  The 1984 and 1985 used the conventional butterfly choke.  1986 thru 1989 used a combo choke/fuel primer system.   They reverted back to the conventional choke system in 1990, WHY ?   A guess is that when these primer chokes work, everything is fine, but as they age, weird things start happening.   Actually it was not a choke as we know it, but merely a system that injects a small amount of fuel directly into the carburetor throat.


This primer system used a knob similar to and in the same location as the butterfly choke knob but in use, this choke knob is actually a dual function choke and primer system.  As you pull the knob all the way out it is injecting fuel into the intake (it's like a syringe and you can feel the tension as it's squirting the fuel into the carburetor) this is the primer function.   When you release the knob it should go back a little over 1/2 of the way in by it's self with the red ring at the base of the shaft still being visible, this is the choke position.  After the motor has warmed up a bit, you have to push the knob in all the way (it snaps in), this is the running position.


Fuel Primer unit exploded view diagram Fuel Primer symbol decal on front of cowling

If your motor has this fuel primer system and you are having problems where it seems that your motor is starving for fuel, or runs better with the breather off, and you are beating your head against the wall trying to figure out what is wrong, look at this type a primer unit.   There is a possibility that if the internal O-Rings are worn AND/OR the plunger is not fully retracted, you may be getting a air leak into the fuel system.   Or it may not be seating when pushed back in, thereby could be allowing extra fuel into the carburetor, flooding it out. 


Carburetor, Early Metal :   Early production motors had all metal carburetors including the fuel float bowl. 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 and got trapped 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 which can get deteriorated over time, especially now with the Ethanol fuel, and 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 which used all steel float needles, the little float needle valve clips did not exist then.  They have become necessary because the 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 on the later carbs, you are likely to suffer from fuel starvation, rather than flooding.


You may see float needles used a nylon tapered tip for the actual shut off seal even on the early production 1974 but these may be replaced parts over the lifetime of the motor.


When ordering carburetor repair kits for these older carburetors, you will find a small white Nylon funnel that is actually a needle bearing.  Somewhere along the line early on, the carburetor was apparently redesigned internally to better support the needle.   So your carburetor may or may not use it, in any case you can not see inside well enough to tell if one is there or not, and if one is, how do you easily remove it without damaging the metal carburetor, so I simply do not use it.  And it is not shown in the illustration below as #21 is the outer rubber needle thread seal.


These older metal topped pre 87 carburetors idle jet needle is 2.500" Over All Length, as compared to the later plastic topped needles as shown below in that section.  The body of these needles were made of brass while the tapered needle itself is made of Stainless Steel.  The straight (front) steel section is .092" diameter and is .300" long.  The tapered needle part begins at the rear edge of the straight part but is .063" diameter, tapering a distance of .240" farther back to it's terminal end of .028" with the end being flat.   Overall length of this steel section is .530".  The overall diameter of the brass threads are .250" with a thread pitch of 28 TPI, with the smaller brass diameter of .156" to where the stainless steel is permanently inserted.


Here is the idle jet needle of the 9.9 & 15hp metal topped carburetors #22 in the illustration below


The illustration below is a OMC carburetor from 1974-early 1987


In the LH bottom photo, you will note that the cam roller pivot arm is white plastic and 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 and 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

1993 to 2001


Carburetor, Late :   One thing to be careful is when reassembling these plastic topped carburetors, DO NOT over tighten this plastic cover, as you could break it.  There are numbers cast into this plastic top that indicate the sequence of the tightening of these screws.


Late 1987 and newer production carburetors up thru 1992 were completely different, had the bottom fuel bowl and 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.


You will find that during this time-frame and later, that the factory was playing /trying to somewhat standardize with carbs.  Apparently the bodies used the same castings but had different throat size for different HP motors from 5 to 20hp, but the covers were interchangeable.  There were considerable top numbers dropped or superseded, ultimately trying to utilize one or two different covers for many motors.  What I have found is that the carburetors up to and including 1992 used 5 screws holding it down, the 93 and later uses 6 screws, with the 6th on the left rear by the roller looking forward. 


Also for the 9.9/15hp carbs up to and including 1992 the idle adjustment screw came in from the front of the carb allowing the adjustment to be made from the front of the cowling.  The 93 and newer used a rear side idle adjustment screw that required the upper cowling cover to be removed to make adjustments.  Other models (6 and 8hp) went to the rear adjustment a few years earlier (1988).  Many of these plastic tops were cast with provisions for either screw location, depending on the final machining, as some had the boss cast at the rear side but not threaded.  Later ones were both threaded, which made that top more universal.  However it seems that there was a early top and a later top.  The early top rear idle screw location was at an angle rearward.  The later tops rear idle screw location is 90 degrees to the carb body.  The front idle needle for the early top is too short to function in the later top.


The covers that had both idle positions threaded, were designed to use either of the needles (front or rear) but only one or the other.  The cover had provisions where both needle passages assessed passage into the same passageway in the top, which in turn led to one hole in to the aluminum body.  Just plug the unwanted hole depending on the model.

The late replacement top covers are made of a different material.  The early covers in addition to the 5 screw holes, had two larger holes at the location of the metal that held the choke rod.  Here there was a plastic standoff that went into these larger holes.  This was slightly longer than the thickness of the top, apparently to not have any extra pressure on the top when the breather was screwed down.  The late replacement covers eliminated these two larger holes, but added raised bosses so that now those breather screws also help secure the top.  In addition to these interchangeable tops, the gaskets also could be different, diverting air/fuel differently as needed. 


In the float chamber here is a white nylon collection box that is sandwiched and gasketed between the main body and the bottom fuel bowl that has a removable main-jet on one side.  These carburetors appear the same for the 9.9 and 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 and goes up into the plastic top for suction to the idle jet itself.  The inner hole here is rather small so the fuel needs to be clean.  However 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, with the guess is that this newer carburetor is probably more efficient.  It appears to have all the screw holes and outboard fittings in the same locations, but whether it would interchange with the earlier versions and 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 and 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 and 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 and 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.


Illustration of exploded view of late plastic topped carbruretors

Also in the newer plastic fuel bowls you can see a drain plug threaded into one side of this plastic bowl.  There may also be another boss on the opposite side.  This could because these parts seem to be as universal as possible, fitting many different carburetors.   And some of these motors do not use a choke, but a fuel primer, if this is the case, this second boss can be tapped and act as a fuel supply for the primer instead of teeing off of the carburetor's fuel inlet line.   These primers inject raw fuel into the rear of the carburetor and behind the high speed butterfly. 



The 1994  9.9hp motor that was my trolling motor for 3 years when I bought a newer Yamaha T8 with power tilt, this older one sat in the corner waiting for a friend to come up with the money to buy it.  Never happened and when I later tried to start it, the carburetor float valve was stuck open, allowing fuel to be pumped into the carburetor and out the vent hole in the upper section.  When tearing it apart, I found greenish moss encompassing this needle, also the same greenish film in the float bowl and on all brass tubes inside the carburetor.  Time for a soaking in carburetor cleaner.  I must not have ran it dry on the last run, but there was a film still inside.


If you do not know any history of the motor, then it may be best to suspect the worst.  Do not purchase a can of spray carburetor cleaner, think that you can spray it into the breather while the motor is running and consider the carburetor clean.  Doesn't happen that way.  It may look like a diamond on the outside but like a sewer on the inside.


Usually you can tear the carburetor apart, clean it up, blow out as much as you can with compressed air and a simple reassemble with good results.  However most of these carburetors have one or more small soft plugs that plug a hole that was drilled in the body to make air or fuel passages.  If you blow everything out with air and the motor still doesn't want to fire or run properly, just maybe there is a blockage in these holes under a soft plug. 


I once ran into one motor that apparently had a watery grave, possibly during a flood because inside the carburetor, the main jet and the main jet passage tube were both plugged with a VERY FINE silt.   I was able to remove the main jet, drill out the plug using the same size drill as the jet was made originally for.   But then after having this carburetor off more times than I wish to remember, it finally hit me that possibly there was a blockage in the main jet tube under this soft plug (the only place I could not see thru).   The price of a new soft plug was only $3.42 where the complete carburetor kit a mere $43.40.  I ordered a new soft plug.   But with fishing season approaching before it came, I removed the old soft plug by driving an ice pick into it then prying it out.   Sure enough, the tube was plugged almost solid as the main jet had been.    On this model the tube had a slotted end down in the carburetor's body, indicating it may have been threaded in.   However in trying to unscrew it, it soon became evident that I would ruin it before it came loose.   So I straightened it best I could, blew the tube out with high pressure compressed air.   Now I can even see thru it.  I then peened the old soft plug slightly, reinstalled it, placed some JB Weld epoxy over the hole that I had made with the ice pick.  It ran so well that I never put the new soft plug in.


Before this it would have been totally impossible for fuel to get from the float bowl into the motor, that is why it would sputter when I squirted fuel into the choke area.  And the previous e-Bay seller said he had just ran it the week before.


Another thing to look at is if you are running 2 motors off the same main tank, like say a 150 hp and a 10hp in all probability the large motor's fuel pump may well pull the fuel out of the small motors fuel line if the smaller motor is not running.  Now when you try to start the small motor you have to pump the priming bulb for some time to bring fuel from the main tank to the carburetor.  In this case, you need to install a anti-siphon check valve in the fuel line at the tank for the small motor.  These are an economical small in-line check valve units that have a threaded fitting on one end, have a hose barb on the other end.   Measure your fuel line so you can buy the right size check valve.  You probably do not need to place one in each fuel line as the smaller motor's fuel  pump should not be powerful enough to drain the other line. 


Also consider if there may be a slight air leak in the fuel line and the motor has sat for a whole that the fuel may have partially drained back into the tank.  When you try to pump up the smaller motor's primer bulb, you pump till you get tired and it still does not pump up.  You may have an airlock at the pump where the only way to get it out is while you are pumping, push in on the ball at the motor's end, relieving the internal air, allowing the fuel to be hand pumped enough to fill the line, removing all the air bubbles.  One suggestion here is to use a nail or something pushed in holding the ball down, while holding this line end high in the air when pumping, allowing all the air to escape.   Then remove the nail, the line should be pretty full.


If the preceding tests do not allow the motor to start, then check the vent on your your fuel tank's cap.  If you are using the o



Carburetor :  1947 Elgin 2.5 hp This carburetor is a Tillotson model  MD 2A.   These carburetors very seldom need new parts other than gaskets which can be made from gasket material.  The entire carburetor usually only requires a complete cleaning as did this one.   These carburetors use a 2 part brass float soft soldered together.  They have adjustable high and low speed needles. 


The next thing was to remove the carburetor, disassemble it, wire wheel brush most of the old paint and corrosion or rust off the outside.  Next was to soak the frozen shafts with penetrating oil for a day.  Once all the stuck parts were free, I cleaned them and soaked them in carburetor cleaner solution overnight.  The butterflies were removed from the shafts, then the shafts were removed and any debris on the shaft removed, cleaned and reassembled.   It is very important to clean the idle tube thoroughly if you intend to troll with this motor.  This idle tube (as seen on to in the RH photo below) can be removed from the top of the carburetor with a screwdriver.  Soak it in the solvent or cleaner and blow this very small passageway hole in the tube out with compressed air. 


Front view of the carburetor, notice the throttle arm adjustment rod screw on the upper right RH side view, notice the top idle screw & the bottom main jet adjustment knob.

The carburetor body gasket was still usable, so were the float, the float needle and seat.  I did not have a service manual, but the float sat at rest, level with the carburetor bottom when upside down, (like specified on other outboard floats) so I figured it must be OK.   The main jet (high speed) needle was bent which needed to be straightened.  This was done using a metal lathe in SLOW speed while lightly tapping the needle in strategic locations until the wobble was eliminated. 


The main jet (lower front) knob is secured to the shaft by an slotted head setscrew and should be preset to 1/2 turn out from bottoming (do not force it here as it mates to a pot metal orifice of the carburetor top).  There is a packing nut on the fitting that the main jet screws into, this can usually be tightened to put more pressure in the shaft making a tight seal.  This knob has a pointer on the top and a bumper type stop on the bottom to prevent over-travel, when installed correctly on the shaft will only rotate about 120 degrees.  The carburetor cover is cut out to accommodate this knob stop.  With the knob indicator pointing UP the cover says RUN, rotate the knob  1/4 turn to the left to START (richer), straight up for RUN after the motor has warmed up, another 1/4 turn to the right to LEAN for idling.


The presetting for the low speed / idle jet on reassembly is usually about 1 1/2 turns out from a gentle bottoming of the seat.


The choke lever as seen in the above LH photo may have the small retaining screw stripped but more than likely missing or someone drilled the hole all the way thru the lever and put a small nail to hold it on like on this project motor.  The hole can be opened up to a #43 drill then tapped out to #4-40 threads where a screw shortened to march the overall length needed.

Also seen on the above LH photo, you will notice the small screw protruding out the brass hex throttle linkage pivot.  This provides adjustment for the carburetor timing and may be out of adjustment.  For instructions on how to do this look below under "Timing Advance".


Tillotson  model MD-2A   (the 2.5 hp does not use the settling bowl)


It is suggested that you also make a new carburetor to manifold gasket.


When you get the motor warmed up and it is running WELL at high speeds in a tank, you may have to now reposition, then secure the high speed knob to indicate RUN.   The low speed idle can then be adjusted.   Move the throttle/magneto lever to slow at about the speed you intend to troll.   Also move the high speed knob to the right, pointing to LEAN.  The low speed or idle jet (top front thru a hole in the cover) can be adjusted by a small flat blade screwdriver.  Adjustment is very sensitive and when it is readjusted the motor should be given at least 15 seconds or more to fully respond to the new setting.  By making infinite carburetor adjustments, and playing with the positioning of the throttle lever, I have had my original motor running so slow that you can just about count the revolutions by hearing the air being sucked in thru the carburetor.


Reed Valves :  Behind the carburetor are 2 aluminum plates held on by (4) #12 screws.  The front plate or manifold, has (2) 1/4" threaded studs that the carburetor is attached to the motor by.   The rear flat plate is the reed valve plate that holds 4 thin flexible spring steel reed valves.   There is a gasket between these two plates.  Inspect these reeds at rest to be sure that they lay flat against the plate.  This is important if you ever want this motor to run well, especially at a slow speed.   In theory, they open because of suction on the bottom side of the piston on the intake part of the stroke to allow fuel to be sucked into the crankcase from the carburetor, then they close as the piston starts down on the exhaust part of the stroke allowing the gas/air mixture inside the crankcase to become compressed for the start of the next intake stroke forcing this now pressurized fuel/air mix into the cylinder bore. 

These reeds are mounted in pairs and held in place by a thin brass cross plate with a single #6 screw on one end of each going thru both the reed and the plate.  If these small screws happen to be seized, DO NOT break them off, but soak again with penetrating oil for a few days if need be.  Carefully remove them then check for any debris that may have gotten lodged underneath them thereby stopping them from closing fully or laying flat against the plate.  Clean and straighten any bent ones, or possibly flop them over.  However the tension (however very minor) should be the same against the plate on all four so that when viewed from the side, you can not see any light under the reeds themselves, BUT with MINIMAL tension against the plate.   Be sure to clean the reed surface of the plate before re-assembly, even band-sand it evenly (lengthwise) if need be to have a flat smooth mating surface for the reeds.


Make sure that both plate gaskets are in good order along with the one between the carburetor and manifold, or make new ones to assure that there is no air leak in this area.

In the photo on the left below note the torn/folded over gasket on the top of the LH photo.  This gasket needs to be replaced.   Also note what appears to be some rust on the edges of the reeds.  The photo on the right, this plate was band-sanded to remove corrosion that apparently got there by coming in thru the carburetor from setting for a considerable time in a damp salt air climate. 


When making a new manifold gasket for the front of the reed valve plate, it may be beneficial to cut the opening slightly larger than just hammering it out off the rear side of the manifold, this will guarantee there is a unrestricted flow from the carburetor to the 8 holes in the plate.


Reed valves shown from the piston side of the plate. Reed plate viewed from carburetor side of plate showing the reed valves thru the round holes

Carburetor Cover :  There is a cast aluminum carburetor cover that is attached to the front of the carburetor by (2) #8-32 X 1" long screws.  The heads of new screws are slightly larger than the hole in the cover, so the heads need to be ground down a bit to fit.   This cover has cast into the face of it START, RUN, and LEAN.   The main-jet knob pointer needs to be adjusted after running to align with these markings on the cover.  This cover was missing on the motor being worked on here, but after looking for a year, I found a good used on on e-Bay, (for a price however) but this was the last piece of this project.



Carburetor ;  These were rather simple carburetors.  The one on this motor was a Tillitson model LMB  229B,  6180612  11-86.  The internal throat is .875" diameter where it mounts onto the manifold.  The one bad thing is getting it off the manifold.  The RH nut has not enough room between the carburetor body and the nut to get a standard wrench on unless it has a short handle.


The float bowl needle seat is unique in that it is a green Neoprene ring held in the hole by one loop "clip" of a coil spring.  The needle then is just stainless and has a retracting clip.  The float is a soldered brass float.  The idle needle is usually backed out 1 1/4 turns.  There is a throttle arm roller that is attached by a eccentric bolt, which is used to adjust the link and sync.  


The choke is simple and attached to the rod by a spring clip similar to that used by the automotive industry.  On the choke plate of the carburetor, it has 3 slight notches that individually engage in a small short standup coil spring that retains the choke in the closest position.


It has a round metal float bowl that is held on by a combo bolt/main-jet (#63) for the 15hp motor and a (#48) for the 9.9hp. 


A long threaded bolt on the left side of the mid/forward of the motor and attached to the timing plate linkage is a slow speed stop.

There is no air breather on this carburetor.


It will be a challenge to clean this carburetor and be able to REUSE the float rubber bowl ring seal.  Mercury's current part number is 27-F10068 which sells for $2.70 but not normally carried by many dealers, however if you go to a lawnmower shop, Briggs & Stratton's corresponding number is 693981 and sells for $2.60.  This seal appears very slightly smaller diameter, so needs to be warmed up in the sun or in HOT WATER or soaked in gasoline, stretch it enough to allow it to stay in the groove in the carburetor's bowl base long enough to install the bowl.  Then it has to be pretty tightly fitting on the outer rim otherwise it may leak.  So trial and error may be the order of the day when using this seal.


A trick here when using the B&S seal, may be instead of installing the carburetor on the manifold, pumping up the fuel, looking for leaks, I found that if you just attach the fuel line to the carb while it is not bolted to the motor, you can connect the fuel coupler to the motor, hold the carburetor level as on the motor, pump it up and if it leaks, a lot less hassle then to re-adjust this gasket/seal slightly.  The problem is the float bowl has only a single thickness of the thin aluminum bowl to seal against the gasket, even a very slight miss-alignment of the gasket can allow fuel to leak out.


Note that this bowl has a flat on one side of the bottom.  This flat goes on the pivot pin (inlet) side of the carb body, but not at 90 degrees, as it needs to be aligned with the float hinge base.


Carburetor LH side view Carburetor front view


One thing that may be beneficial to add, is since the carburetor retainer nuts are 1/4" X 20 TPI (course threads) and hard to really tighten in the positions they are in, you might consider adding lock washers under these nuts at your final assembly.









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Originated 04-01-2015, Last updated 07-07-2017
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