You need to do some tests, which are quite easy. How are the needles set, if you are running extremely rich it could react as you describe. Try changing your plug wires and see if the "bad" cylinder changes and follows that particular plug wire. If it does, you very likely have an ignition problem. If the "bad" cylinder stays the same, try changing plug, or at least check color and gap and adjust. Check for any leaks around cylinder to crank case gaskets, they will show as being dirty. If you are unable to find and fix the problem alone, seek the help of knowledgeable people in your area, and if that fails, send the engine to your distributor for a check.

DKjens