Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Well, weak ignition is surely a possibility. But the only thing that steers me away from the ignition is your comment about being able to keep it idling OK by tickling the choke. Needless to say, your lack of power at high speed might be a problem not related to the poor idle, weak ignition is surely a possibility…Doesn’t take too long to check grounds and do basic ignition meter tests, so that is always a good idea.
I remember that waste spark stuff, nice to see other manufacturers shooting themselves in the foot with cost cutting crap, not just OMC. We had that system on some large stern drive engines, slowly had to change them all back to conventional distributors. Had the waste spark on early four strokes also, learned to say that backfiring on acceleration was a "normal" condition for these engines….
OK, the carb butterflies are working as the should. I looked at the online manual you posted, seems like kind of a generic one, but not familiar with the way Yamaha lays out their service literature….So, I’m not sure I completely understood the synchronization process for your engine.
But, I did notice that Yamaha seems to control their idle speed on some engines by "cracking" the throttle plates open to increase idle RPM….So, my comments about both throttle plates being completely closed at idle don’t apply to your Yamaha. Did you check TDC? Don’t forget to properly set the idle and WOT timing. If problems are still present, temporarily swapping carbs will help steer you in the right direction.