Could be a plugged needle valve. Have you tried spraying some cleaner into the carb with the air cleaner removed? Try removing the air cleaner, get you some carb cleaner and spray it in the carb while the engine is at idle. This should remove any buildup, and there is buildup, guaranteed, inside the needle valve assembly allowing the machine to run smooth. Also might want to remove the bowl on the bottom of the carb and clean it too.