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
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.