I have used "BERRYMAN" fuel system cleaner, and its cheap and works. If you have a stuck valve, your in trouble. I had one on my 47 cessna one day. Landed at an airport, took off the valve cover, trickled gas on the valve stem while tapping the valve stem with a hammer. It freed right up. If you have problems like this, learn to change your oil on time, and youll stay out of trouble!