Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Yes, if the reeds aren’t sealing, the primary compression/crankcase pressure isn’t being developed and won’t be forced into the combustion chamber. And yes, the cylinder won’t fire properly and will appear wet/washed, but with fuel, not necessarily water. My only question is whether or not it is normal for the reeds to show a bit of light through in the static position…This isn’t uncommon for some of the OMCs, the reeds seal up when the piston is on the down stroke and pressurizes the crankcase. You have had the intake off, can you see the cylinder walls at all? It is possible that the debris got caught on the exhaust side of the cylinder wall gouging it up, this would create a primary compression/crankcase leak as well, this might not affect the compression readings you are seeing. Let’s hope this is not the case.
Not being familiar with the Yamaha linkage set up, can’t really comment on the methods you described. But, there is one thing I know which is universal to any carb….Just because the linkages are disconnected, this does not ensure the carb butterfly is closing completely. The throttle shaft could be jamming, or the butterfly could be jamming/not closing completely, perhaps due to debris from the initial failure ending up in the carb.
The only way to ensure the butterfly is closing completely/properly is to remove the carb and look at the butterfly after snapping it slightly, the return spring could be weak as well. It sounds like you don’t have the Yamaha manual with the sync instructions, let’s hope someone can help out by posting these pages.
Once sync has been set up properly/confirmed, swapping carbs will be the next step….