a 2 stroke engine burns oil by its nature so the engine will only need a piston and boring if the old ones are damaged which would result in bad compression and not an oily plug.This is a symptom of a 4 stroke engine being in need of a piston/rings/rebore.i would say that the carb is certainly flooding and washing the carbon off of the piston crown and combustion chamber and onto the plug which is why its black and oily.so you need to take the carb off and make sure the float needle and seat are clear of debris is in there ,also check the condtion of the float needle tip for any wear and make sure the float is not full of fuel as it will not float and stop the fuel flow if its leaking.i would also say the vacuum tap may be faulty as it the fuel should only flow from it when there is a vaccum pulled while the bike is running.does it flow from the fuel pipe without the bike running?pull it off th carb and have a look.
adam bike doctor, February 2010
