HEllo and I think I amy be able to get you started in the right direction. If the smoke is black that means it is unburned gas, period. If it has a carb, I would start with the carb and see if the floats are stuck or set wrong. The engine is accepting the gas and can not burn it. Perhaps a fouled plug, burned head gasket, or damaged valve. A compression test will tell you just about all you will need to figure out the problem. If one of the spark plugs are black and wet than that plug may not be firing and gas just passes through. If it is fuel injected, a bad injector may cause the problem. I hope this helps you get started, good luck, geo
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