Mo need to buy a jet. take the top off the carb, and there should be a slide valve there, with a silver pin on the bottom. If there isn't, ignore my advice. Take out the silver pin. You will see notches at the top. There will be a c-clip in one of the slots. to lean the mixture, move the clip up a few slots. do a plug chop, and keep adjusting the mixture un till you get it right or run out of slots. (then it's time to buy a new jet.) To make the mixture rich, reverse my procedure. move the clip down.