Carburator Issues

Have Tryed disconecting the Tacho and then running over a known course and see if you get a speed increase on you log? Have you tryed checking The revs of the Motor with a workshop Tacho?

 

I am just about getting to the end of any more ideas here. If it were my boat I would rev the balls of it and be shure that it will and wants to rev over 4000 It may be that it just bogs down at 4000. I have seen this on cars where they wont rev without the air filters on. Has it got air filters on? Is it mechanical secondaries or vacuum. can you look down the carb at 4000rpm make sure the secondaries are wide open and that the choke is not closing.
If its vacumm check the pipe. I remember some how converting vacuum into mechanical by using a small nut and bolt in the arm operating slide.

I am sure I do not need to point out that working on an engine at full throttle is dangerous beyond belief.

 

Test a couple of valve springs. I know you said the engine has never been over-revved, but if nothing else works out, test the valve springs. If they're weak, they could be holding the engine back.