I have owned a boat with a 2GM for the past 4 years or so. It has always had a major vibration between about 1400-1800RPM, but only after I have run for about 30 min, and under load. If I increase throttle to 2000 RPM very suddenly smoothes out. If I put it in neutral it is fairly smooth. When leaving the dock for the day the vibration/noise is there, but very minor, it is only after running for about 30 minutes, reduce the RPM and then increase the RPM again that the noise/vibration/roughness comes. I have aligned the prop shaft. Put in a flexible coupling. Adjusted valve clearances. Replaced the prop (the old one had very poor prop tip clearance). I replace fuel filters every season. I thought I had it the other day as I discovered that the mounts were reversed on the one side, but after correcting it it is still there. I have tried several tests while tied up to the dock, but it seems to be impossible to duplicate the fault this way; I seem to have to actually take it out into the bay. Reverse is similar but not as bad. Help!
Rudy
Rudy