Re: 1974 Johnson 85hp thermostat
To expand on what was just said, look for a thick hose coming off the bottom of each cylinder head. Those hoses go to a squarish box--the thermostat housing. Remove the hoses, take out the two screws and remove the thermostat housing. The brass thing you will find inside is the vernatherm -- fancy name for thermostat. There is also a plastic valve in there. And there is a pin in the small end of the vernatherm -- don't lose it.
I'm not sure, but you might have to remove the lower cowling cover to get at the two housing screws. You'll know when you look at it.