Hey Bruce,

My boat acted the same way last time out. I'm thinking carb icing (and/or manifold) though. I have CHT's on all four cylinders. they were below 200C, so I don,t suspect overheating.

I researched carb icing a bit, and found out it is most likely to occur about 70F at high humidity (like maybe over water?). Also if you have a rich mixture.

I touched my manifold and nearly froze my hand. I know the old VW's would freeze up the intakes if that exhaust pipe attached to them got plugged. They act just like what you said.

I'm making manifold #2 now, hoping to fix that problem.
If it doesn't work. I may have to rig up an aircraft style carb heat system.

BTW, what is a psru, is that a re-drive by any chance?
OFTA, they are spendy, and that sounds like something my swedish wife would say

I am sure that boat came new with a 1500 VW engine, and I don't think the rpm's you mentioned are too much for it. I think you should think about timing, and tweaking the mixture by trying different jets etc. JMO

I would also like to give you a hand, but I got one of my own I can't seem to get right.

Dave