I am helping a friend with his boat he had just bought. It has an 88' Yamaha 90 on it. It is doing a very strange thing. When we take it out it will run normal to 5000 rpm. Then after running top speed for a few minuets it starts to like misfire or breakup to the point you have to back it down to idle. It then recovers and you can take it back up until it does it again. Now if you run it at no more then 4000 rpm it runs all day long. Go to 4100 and up then back down to idle you will have to go. Once it starts you can't get it to stop without lowering the throttle to idle. The fuel pump diaphragms look like new. A new water fuel separator was put on. Plugs look good. While running fuel always appeared in the little plastic bowel on the engine. I tried to squeeze the bulb while it was acting up and it didn't help. The engines appearance overall looks like new, all wiring and hoses like new. I don't think this is a fuel problem due to it running perfect and then being able to repeat the problem at over 4000 rpm. What do you guys think? Any help would be appreciated.
