A wet and black spark plug generally indicates oil fouling. I'd check the compression on it following the procedure in the service manual... AFTER adjusting the valves.Hello! My 420 started smoking from muffler, white to gray smoke. Pulled plug and it's black and a bit wet. No oil in muffler. Oil and coolant levels remain the same. What could the issue be?maybe valve gasket? Any help very much appreciated.
If it was coolant washing the chamber down, the spark plug would begin to wash clean and develop a bronze colored tint on the center insulator.
Has this motor been submerged before?