Does it do it after the engine warms up?
Have you tried using a different brand of oil or spark plug?
Have you checked to see that the needle is not lodging on the spring. If the spring is not pushing down on the silver washer, on top of the c-clip, the needle may be lifting too high or sticking, causing too much fuel to release! If that is the case, its an easy fix. Just reposition the small parts and check operation.
Try using Sick Bike Parts aluminum gaskets, and the rubber o-rings between the carb and inlet tube.