What
RPM are you running at? Have you ever run at a higher
RPM in flat water?
My guess is your
cooling system is very marginal right now and in flat water (lower load) it is keeping the
engine mostly cool. The increased load of driving into the waves might be making just enough extra heat that you are
overheating the engine.
If you have an IR thermometer next time you are out check the temperature of the
exhaust manifold and the
head, check at both cylinders. I suspect you will find that you have hotter spots near #2.
The MD7A has a tendency to clog up the water
cooling passages in the exhaust manifold and esp. between the exhaust manifold and the
head. Pulling the exhaust manifold to clean it out is likely in your future. If you have access the job isn't terribly hard, just time consuming.
I'm in the process of putting my MD7A back together for the same reason. The exhaust manifold was completely clogged shut so no cooling water would flow through it and into the block. Ended up boiling the manifold and also soaking it in citric acid to clean its passages out. The head was only clogged right where it mates to the exhaust manifold.
While I'm at it I also built a new exhaust riser, will be converting to fresh water cooling, getting rid of that horrible metal pipe between the
transmission and
water pump and altered the cooling flow/path to take advantage of fresh water cooling.
Shawn