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The fuel pump on the motor is pulse driven by the stroke of the piston. Which makes the fuel pump pull fuel from the fuel tank and push fuel to the carbs. If you can squeeze the fuel primer bulb and the engine runs better you have an air leak from a loose fitting, a restriction in the boat side fuel system, clogged fuel tank vent or a bad fuel pump. To check for boat side fuel issue run the engine on another fuel tank. If engine runs better then you have a restriction in the boat side fuel system. If no change the fuel pump is failing and will need replacement. These are the only things that can cause your symptoms. Just because one engine runs on the boat doesn't mean the other will. This engine uses more fuel than the other creating more vacuum on the fuel system that the smaller engine. So just do these tests to eliminate all possibilities. Nothing is impossible.