hi, i asume that you have checked that the air filter is clean and that you are running a bp5es or 6 ?. check how many turns the air screw is out and wind it all the way in and back out 1 1/4 turns to start with . warm the bike up, if it seems to be running ok ride it up and down a few times so it clears the motor out i.e. give it a bit ,then while riding pull the clutch in and hit the kill switch so that the motor doesnt have time to idle. take plug out and check colour, a nice biscuit colour in the middle is ideal. if it is still black turn the air screw out another 1/8 of a turn at a time till you get that biscuit colour. hope this helps .