Don't think a bad plug, does sound like fuel delivery maybe. A few of the normal tests will find it. I hope it's not the beginning of a timing gear/chain slip. (that's the backfire part). But could be carb/choke thing. Not enough concrete info yet to determine what of many things it could be. Time and/or testing next. Do you know the procedures/tell tales?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn