have you re built the carbs? if not ,clean them out with 1 can of carb cleaner. spray all the high and low speed jets whitch are the tbes in the carb thay usually have a sllot for a flat head screw driver take them out and clean. the high speed jet needle valve is controled by a set of diphrams located under the crom caps on top of the carbs make sure thay dont have anny cracks in them and thay are sealed good ,