M15D2 Excess Fuel in Lower Cylinder

[tmccaffery]

My 5 year old M15D2 is getting too much fuel in the lower cylinder which is causing the spark plug to foul, low power and problems at idle. My mechanic has overhauled the carburetor and fuel pump but this has not corrected the issue. While we are scratching our heads over this issue any advice or suggestions would be appreciated.

 

[pvanv]

Did you verify ignition?

 

[tmccaffery]

The mechanic said he has, but I will double check. He has changed out all of the electrical end (plugs, coil & etc.) except for magneto with no change. The engine starts, quits when it is in idle for longer than 10 seconds or so and won't develop full power at full throttle. The upper cylinder and plug is clean while the lower cylinder/plug is dirty.

 

[pvanv]

What exactly did he do to the carb?

 

[tmccaffery]

He cleaned and re-built the carb with the carb rebuild kit, as well as cleaning & etc. the fuel pump, and says that it is working correctly now. He is currently thinking that there might one, or both, of the inlet manifold gaskets may be leaking.

 

[Mc Tool]

Broken reed valve petal ?

 

[tmccaffery]

We looked at the reed valves yesterday and they are intact. The only odd thing is that both cylinders are firing at the same time about every second or third firing cycle. Since the exciter coil has not been replaced I have one on order. Also, the lower crankshaft seal may have been leaking as some oil was found in the engine base. Unfortunately, due to the approach of Hurricane Milton he's not going to be able to do anything further until next week, by which time I hope the new exciter coil is on hand.

 

[Mc Tool]

Both pots will fire at the same time . Its called wasted spark . Rather than have a sensor for each pot they have only one and it fires the (dual )coil every 180 deg with one plug sparking harmlessly at BDC. When testing for spark both plugs need to be earthed or neither of them will spark.
I am just resurrecting a M18d ( got it for free ..... which was way to much )

 

[tmccaffery]

Thanks to everyone for your suggestions. The problem has finally been solved by replacing the carburetor jets. It turns out that the main jet was heavily eroded to at least one size bigger than it should have been. After replacing the jets the engine runs fine. Nobody thought to check them during the carburetor re-build due to the engine's age and low hours.

 

[ct1762@gmail.com]

Thanks to everyone for your suggestions. The problem has finally been solved by replacing the carburetor jets. It turns out that the main jet was heavily eroded to at least one size bigger than it should have been. After replacing the jets the engine runs fine. Nobody thought to check them during the carburetor re-build due to the engine's age and low hours.

how on earth would brass erode with low hours all the way through to effect fuel flow on only one cylinder?

 

[tmccaffery]

how on earth would brass erode with low hours all the way through to effect fuel flow on only one cylinder?

My mechanic and I wish we knew the answer to that one. Had we even suspected a carburetor jet issue I would have saved a lot of mechanic time and my money on fixing it. Since the hole in the jet was also angled to one side, my only conclusion is that the main jet casting had a bubble in it that had a very thin surface where fuel was flowing. Once that thin layer eroded away the bubble opened up creating the hole that we found.

 

[ct1762@gmail.com]

very strange! i say that because at idle, its not drawing ANY fuel from the main. that kicks in around 1800 rpm or so. all a larger hole would do is make it stall/bog when you gave it some throttle. the excess fuel problem must be from a bad needle/seat or improper float height. literally the only way it can flood like that

 

[tmccaffery]

very strange! i say that because at idle, its not drawing ANY fuel from the main. that kicks in around 1800 rpm or so. all a larger hole would do is make it stall/bog when you gave it some throttle. the excess fuel problem must be from a bad needle/seat or improper float height. literally the only way it can flood like that

I agree that it is strange. All that I can add it that it did exhibit the stalling/bogging that you described, most frequently at start up. Anyway, the jet was eroded as I noted, the engine ran fine with new jets and any future problems belong to the engine's new owner.

 

[Mc Tool]

My feeling is that there may have been another issue close to the main jet ....like maybe it wasnt tight or sealing around the base of the jet allowing extra fuel to bypass the jet , or a blocked air corrector jet. If there is raw fuel ( as opposed to atomised fuel ) flooding the inlet it will tend to go more to the lower cylinder.
My ( M18d ) carb had the main jet corroded in and someone had mullered the screwdriver slot so I bailed and bought a Ali ex carb ( it is identical to the original ) for 30 something bucks .
Awhile ago I did a bit of research ( I read up on others work , not actually do it myself ) into fuel flow thru jets ( weber , mikuni ,delorto ) and basically there is more to fuel flow control than the size of the hole in the jet . There is every chance that a jet with a corroded orifice will flow less fuel due to turbulence .
Pleased you got it sorted

 

[ct1762@gmail.com]

My feeling is that there may have been another issue close to the main jet ....like maybe it wasnt tight or sealing around the base of the jet allowing extra fuel to bypass the jet , or a blocked air corrector jet. If there is raw fuel ( as opposed to atomised fuel ) flooding the inlet it will tend to go more to the lower cylinder.
My ( M18d ) carb had the main jet corroded in and someone had mullered the screwdriver slot so I bailed and bought a Ali ex carb ( it is identical to the original ) for 30 something bucks .
Awhile ago I did a bit of research ( I read up on others work , not actually do it myself ) into fuel flow thru jets ( weber , mikuni ,delorto ) and basically there is more to fuel flow control than the size of the hole in the jet . There is every chance that a jet with a corroded orifice will flow less fuel due to turbulence .
Pleased you got it sorted

good info. i would add that i am not always correct about saying the main jet is the sole contributor for midrange-full throttle. on a few carbs, the idle circuit is always active, ie. "assisting" the main jet. i dont think these are like that just thought it was worth mentioning!