Black crap on the transom is typically caused by two things.
First, a fat idle mixture, bad power valve or high float level causing the motor to load up at idle.
Second, retarded idle timing causing a late fire and causes the carburetor throttle played to be in transition from idle to partially open to get it to idle fast enough, this make a hot exhaust that's a little fat, it cooks the carbon right to the boat.
Find out where the timing is at idle and pick up a carburetor kit and a few power valves (like a 4.5, 5.0 and a 5.5") and go through the carburetor, ensuring the float levels are set and the ports are all clear, it could have plugged up air bleeds.
Good luck.
GT
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