When no after-market parts, like exhaust pipes, have been installed usually the cause for backfiring is air leaking into either the intake or exhaust tracks. It could be the rubber intake manifold clamps need tightening, or perhaps the various exhaust joints need tightening. I would tighten everything and see what happens. If that does not do it, it could be cracks in the rubber components such as the rubber tubes between the carburetors and the heads.

Fred