Mine was doing this and I just fixed it. The Diaphragms at stuck in the carburetors. Pull the cover off the right sides or the carbs, clean and make sure the boots are good. Clean the needle valves. (Car/choke Cleaner is all I used) Clean the inside of the carbs while the diaphragms are out. Make sure the diaphragms move freely when you re-install.
After doing this mine started right up and ran perfect. Good Luck