Remove the carbs and clean them. Be careful not to tear the diaphragms and you won't have to buy new ones. Use carb cleaner to clean all the journals too. There are 2 small fine screened filters internally that need attention also. If you get in a jam give us a shout back. Clean carbs should help a lot. You might want to do a compression test too to see what the engine is doing internally too. Compression should be ideally at 150 psi.
Karl