OK, I think I've got it. I've been paying more attention to my starting technique, and I think I was pulling the throttle back a little just as the motor would catch. I've made sure not to do that now, and I haven't had a backfire in two days. Windrat, thanks for your suggestion. You've reminded me that I need to reseal the pipes. A close examination this evening revealed an audible leak on the left pipe. I don't think it's enought to warp the valves or anything like that, so I'll wait for the weekend to fix it.
Thanks all!
