I was just getting ready to type in to replace the in line oil check valve between the oil pump and the main fuel line to the carbs.
The way to check the crank case check valve is to run the engine on a hose or in the water, wait five minutes and loosen the fill cap on your inboard oil tank. If there is pressure released when you open the cap, replace the crank case check valve on the starboard side where the 2nd oil tank hose connects at the engine block.
The problem could also be a torn or split diaphram in the pulse fuel pump. You can check that by removing the fuel pump from the side of the block to see if there is fuel present from the hole in the rear of the pulse pump after pumping the primer bulb prior to the pump removal.