Could be the old gas, drain it all out and put fresh gas in. You could try Seafoam, just follow the instructions on the bottle. I've heard mixed reviews of it, never tried it myself.
It's quite likely that old gas has gummed up the carbs though, and a real cleaning is the only reliable way to remedy that. @
ducatiman provides a very well reviewed cleaning service, he'll make your carbs look and run like new.