Re: 1964 Merc 350 question

[David Stanton]

Check first to try and find out what is stuck. Betty Ann mentioned the lower unit. I’ve seen a broken vane on a water pump impeller stick an engine. I currently have one with a mouse nest in the lower cylinder.
To check for stuck pistons/rings, take out the spark plugs and put a long screwdriver or other metal bar into the spark plug hole until it touches the piston. Hold the screwdriver with one hand while you try to rock the flywheel back and forth with the other hand – even if moves little or none. If only one piston is stuck, often you will feel the other piston move slightly as a "tick" on the screwdriver. If you feel it on both pistons, look elsewhere for the jam-up. If the driveshaft is totally seized in a bad bearing, this method probably won’t work work.
Often you can move the flywheel back and forth some when pulling the starter rope will not move anything, and rocking it back and forth can help unstick pistons.
Dave