Backfiring can also be from running to lean. Had that problem with the single barrel on my 292. Rev it and pow pow pow! lol! Had to pull the mixture screw out 1 full turn and it ran ok. You may have your light set wrong....but it won't backfire with a little to much advance. Retarding it to much with cause major hesitation and backfiring...too far advanced it will be hard to start or won't start at all.
