your poppet valve opens around 2500 rpms or so. it lets more water in the powerhead above that speed. if you run your motor just below the speed that it opens your motor will run hot. if you have a water pressure guage you can watch as you slowly accelerate the pressure increases than drops(poppet opens) than increases again. long story short... you may need to replace the water pump. Merc pressure style water pumps like yours should be changed every other year. they are cheap enough and very easy to replace.
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