Hi b john,the problem sounds very much like its runing too rich the excess black smoke is the hydrocarbons coming out of the exhaust as the engine is not burning all of the fuel that is going into it,the reason could be one of many including the main jet being loose or it may have fallen out into the bottom of the float bowl,the air filter may be heavily blocked therefore creating a high vaccu in the venturi that is sucking too much fuel up the main jet ,the slide needle may have got stuck up and is maybe allowing the main jet to be wide open,the float may be maladjusted or faulty too ,does the carburettor leak any fuel ?the excessive carbon will foul the insulator nose on he spark plug and cause a misfire too by the way.
adam bike doctor, April 2010
