i own 3 iwata airbrushes, best guns i own, but i wont own a siphon feed because of the sputtering problems. if yours is gravity feed (cup on top) it maybe because your gun is dirty, try what ram jet suggested soak the gun and make sure all passages are clean, it also could be your still too thick on your paint mix. also depending on the paint your using if it is high metalics or pearls they can cause clogging