1985 Yamaha 150 2 stroke - power loss in mid range RPMs

[rburling]

I rebuilt a boat a couple of years ago that had been sitting in a storage yard for over 10 years never run. Rebuilt all the usual suspects (carbs, oil pump, hoses, plugs, etc). Everything is running great now except that I have a low spot where the motor seems to sputter and lose power between about 2,000 and 3,000 rpm. Below and above that range, the motor runs great. Problem is that is the range that is a good speed for some of the rivers I fish. Rebuilt carbs, adjusted fuel mix and reset timing trying to correct the issue. Even had a local boat mechanic double check everything. I am stumped...any suggestions on what might be causing this?

 

[99yam40]

Re: 1985 Yamaha 150 2 stroke - power loss in mid range RPMs

I would try monitoring the spark on the different plug wires with a timing light at the problem RPM to see if it is loosing spark on any of them, and make sure the timing is staying where it is suppose to be while you are at it. Or you can actually monitor the primary voltage to the coils at the problem RPM if you have the proper equipment.

If that is all good then it is back to the fuel/air ratio(carbs) in my opinion

 

[rburling]

Re: 1985 Yamaha 150 2 stroke - power loss in mid range RPMs

Thanks 99. replacing impeller this weekend and then I will give that a shot. If I remember right it only loses power under load, so may have to try this on the water.