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I had a similar overheat issue with a 1987 70 HP Mercury outboard. After doing all the easy things like water pump thermostats and poppet valve the problem persisted. I ended up pulling the power head off which was a chore since this was a salt water motor and many of the bolts were frozen. I also needed to remove the exhaust adapter plate, upper motor mounts, exhaust tube and quit a bit of other stuff to get to the gaskets and upper water tube seal. I replaced the large gaskets that sit below and above the exhaust adapter plate (took hours to clean the surfaces of the old gaskets off). I also replaced the upper water tube seal and the exhaust tube seal and any other rubber part while I was at it. I replaced all the old bolts with new ones. After getting it all back together success. The engine doesn't overheat anymore but boy was this a job. It took me many weekends and I lost a whole summer but learned quite a bit along the way.