Its almost certainly blocked jets. Take off the cover to the carb and then carefully see how many turns each of the two jet adjustments ( not the idle adjustment) take to screw in as far as possible. This is to store current settings. Then take out each jet screw and clean the whole thing with carb cleaner spray. Reassemble by screwing in each jet as far as it will go then unscrewing as per your calibration. It should now work fine but if not, use set up instructions that you can find on many forums.
Doctorpete
July 2010