rpm restriction

[papajoeky]

I have a 1978 150hp. I have had the boat for one season and do not believe it is getting to full rpm under a load. I put a new tach on it and at idle it shows 800-1000 rpm. In the water not in gear the engine will rev up past 4000 rpm but in gear it will not get above 3200 rpm. With the engine trimmed down at full throttle the boat is turning 2950 rpm at 36 mph. With the engine trimmed up it is turning 3200 rpm at 46mph. The carbs are opening fully, the power packs are working, all coils are working. I am running a 23 pitch prop and there is no change in rpm when I change to a 17 pitch. I have been told it is possibly an exhaust restriction or a reed restriction. I was wanting to see if that is possible before I pull the power head off to clean it. Thanks for any help.

[boats.net]

Verify that your timing advance components are working properly.

[papajoeky]

I have verified that the advance is working properly, the carbs are fresh rebuilt, the electronics are all working. I have talked to a few mechanics and no one has any idea. The boat runs great but it will not get to full rpm. The only thing I haven't done yet is check the gears in the lower unit. Still needing any help I can get.

[boats.net]

The RPM should have changed when you dropped your prop. I am stumped as well.