Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Joe
I think you are right on track, with your foam theory. It surely seems possible that a piece of insulation got sucked in a carb and jammed a reed partially open. And YES, a leaky fuel pump diaphragm can spill excess fuel into the crankcase it is connected to. Is the fuel pump pressure port on the top #1 cylinder? If so, you could run the engine with the fuel line disconnected, the top cylinder will kick in for a few seconds once the fuel is out of the pump but there is still some in the carb bowl. But again, beware that engines usually lean out a bit and pick up RPM as they are running out of fuel, don’t confuse this normal situation with a leaky fuel pump. I suppose you could just bypass the fuel pump and connect your fuel tank hose to the line running from the pump to the carbs, then simply pump the fuel bulb on the tank to keep the fuel pressure up, this will allow you to run the engine longer and evaluate this theory better. Please do not go ripping into that fuel pump looking for a problem unless this test confirms the fuel pump is at fault.
You have had this engine all apart, so pulling the intake manifold back off doesn’t seem too difficult. Pull the air box and carbs first, looking for pieces of insulation that might help confirm this theory.
You can do these tests with the boat on the water, just don’t be a clutz like me and drop pieces in the drink!
I hope one of these two theories proves to be your issue….