I don't think what I'm about to share is the cause of your backfiring but others reading this might find some insights into their problem with with this issue. Yesterday as I was riding my Ace it started sputtering and backfiring, it got so bad I had to pull over. I first checked the fuel by turning the valve to reserve as the engine acted as if the gas had ran out, no luck in that. I then gradually revved the engine and found if I did so slowly it would regain some power. I did this for a few minutes and then rode home sputtered and backfired all the way. My neighbors walked over as I pulled into the driveway, and offered advice. "sounds like bad gas....sounds like your plugs need'a changing" etc. I was not convinced. I shut the bike down for a few minutes and then tried to restart it, no luck. Now it would not turn over and the lights were dim. After explaining to one neighbor that it was impossible to "push start" a bike like this, I asked a him for a jump start. This brought the bike alive and running well as long as the jump cables were connected. Getting there I thought, I ran into the house for my Volt meter, and checked the battery. The Voltage ran 10.2 volts. I had lost a cell in the battery while riding along at 50mph. This surprised me in that the battery was two months old. I replaced the battery and the Ace has been running fine. I also ran some voltage checks to see if the bikes charging circuit was doing its job and that it was not overcharging, it was fine. Moral here is that there can be several things that cause backfiring, in this case it was a kaput battery.