Once you have the gas valve disassembled, take a small screwdriver and pick and scrape out the built up carbon, then take a torch tip cleaner (Pictured. You can get one at any hardware store for a few bucks) and file clean the gas ports. I feel that the gas system being difficult to clean as some say is highly overblown...it's really quite simple and a very well designed system that doesn't need to be cleaned very often.
!/8" drill bit(long) and a pair of vice grips. Use to clean gas valves. Keep whole front section dry(no oil). Oil just carbonizes and gunks things up. Clean pistin and rod with fine steel wool. Put 3-4 drops of oil on the groove in the barrel where the bolt slides back. It is right inside the loading chamber. Preload the second shell onto the carier by pushing the little tab at the back of the floorplate after loading it into the mag tube. This recipe keeps my 303/390/391's good for 2000-3000 rnds. When the forend cap gets too tight to unscrew by hand use the vice grips and an old tee shirt to loosen.