I had the same problem and after trying every thing else I purchased a gallon can of carb cleaner from an auto parts store. This is the kind that comes in a paint type can and has a basket inside. The carbs have to be disassembled and all plastic or rubber parts removed. Soak the carbs overnite and wash them with water then blow out all passages with an air hose. This is a commen problem with these bikes and it must be from some tiny orfice being plugged