got a 91 electra glide. when starting, i pull out the choke, let warm up and drive off. i get a backfire, almost like the bike is starving for fuel. i've made sure the petcock is all the way open, and wind up giving it a little more choke. after the bike warms up it's fine. i tried some seafoam in it, and it seemed to help until the next tank of gas. any ideas?