Gray/black smoke is a sign of overfueling. Generally, when it is gray, that means that the fuel is not burning efficiently for whatever reason. A very common cause of this is worn out injectors that are popping off at too low of a pressure and have greatly enlargened holes. If you can find someone local with a pop tester, it should not be expensive to have them test it.
Taking some pitch out of your propeller is definitely a good idea.