First thing I´d try is running a can or two of seafoam through it. Then probably spark plugs and wires. If it´s still acting up after that, the carbs probably need to be rebuilt. Gasket kits aren´t too expensive, and those tiny carbs aren´t too horribly complicated. Just take your time, be gentle, and follow the book to the letter. You could probably remove them yourself and then get them rebuilt for not too horribly much as well. Most of what a garage would be charging you for is getting the carburetors out of the bike.