I have a mid 70's 302 V8 Ford Mercruiser motor. The engine runs fine at idle and also if you slowly give it gas. But when you hit the gas hard or are on a load in the water it will pop and backfire through carb. Also sometimes will stall after running in water for a while. I have changed the distributor to an HEI instead of points, and have also changed the cap, rotor, coil, checked spark plugs and wires and seem to be fine. I am running out of things it could be. If anyone has any ideas please respond.