Since a sticky valve only needs to stay open about the thickness of a strand of hair to cause a backfire, just watching them move doesn't accomplish much. At normal idle speed of 500 RPM or so, each valve is opening and closing approximately four times a second. The chances of catching one actually failing to close all the way and causing rough running or backfiring is pretty slim. Gummed-up valve guides caused by a dirty fuel tank usually happen after the engine has warmed up and then cooled down, and a filter between the tank and the engine doesn't seen to help much. I'd suggest using a clean auxiliary fuel tank and adding a healthy dose of lacquer thinner or some other type of strong solvent to the fuel for your next run, and be sure the valves are adjusted slightly loose, then run the engine for several minutes at a fast idle if possible (750-1000 RPM) . A little valve noise is better than getting valves stuck and possibly bending them. The intake valves and pistons are a close fit, and a seriously stuck valve can come into contact with the top of a piston. It's impossible for pistons and exhaust valves to meet because of the shape of the combustion chamber and the angle of the valve.
Jerry
