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Is there any concern about gasoline fumes when you crank the engines over with the spark plugs removed to run a compression test? (I have done lots of these on my old British sports cars and, before that, on VWs, but never on a marine engine.)

thanks,

Pschauss,
I you have any doubts about gas fumes, run your blowers or set up a fan to keep air flow around the engines. The open engine boxes provide plenty of air circulation around your motors with blowers and fans also close your fuel line shut offs at the tank.
I have done 100's of compression tests without ever having a problem. All cylinders should be close to each other in readings on gage after testing each hole, if not you found the problem.
Careful removing the old spark plugs they have a tendency to break off at the ceramic if they have been in the boat for a long time, spray with Corrosion X or other lube before you even start.
Pete Fallon

Use a remote crank switch, amazon, local auto parts store.

Leave the ignition off and one end of switch goes to starter batt, the other to yellow wire on starter which is crank. Make sure throttle plates are open when conducting check.