Those lower units are easy.----I was repairing them at 18 years old as mechanic at a Johnson dealer !-----Is water 6 or 7" above the impeller in your test bucket ?----Thermostat in place ?
Is the gasket between the carburetor and block the correct one ?----Some have 1 wee hole for the crankcase PRESSURE pulse.----It must line up correctly !
What are the compression numbers today ?----Water pump impeller replaced ?-----Overheat horn tested ( if equipped ) lately ?----Spark on each cylinder that jumps a gap of 3/8" or more ?