More info needed. How loud is the backfire? Has it recently had a good tune-up? Plugs & wires, valve adjustment, timing, points, condensor , advance diaphram & base plate functioning, carbs tuned & float levels checked? What color are your sparkplugs? Whew, I think I got it all. Oh Yeah, Confirm that your carbs are round top SUs. My uneducated guess is, it's caused by either wrong timing or rich mixture or both but you have to rule out everything else to be sure. I vaguely remember my 71 stumbling at idle now & then but I don't remember it being a backfire or at least a loud one. How strong does your engine run from 700RPM-5000RPM? Any stumbling, low power, backfiring on or off the throttle? OK I'm done. I think. Mark in Portland