If it ran OK then started to backfire, I would repeat the jet cleaning process. Often times, those idle jets will clog up again right after cleaning. Replace the fuel filters to make sure that no crud is reaching the carbs.
Check for exhaust leaks. Cold air mixed with the hot air exiting the heads will cause a backfire. Also, go to the auto parts store and purchase a can of ether (starting fluid) With the engine running, spray the fluid near the base of the intake manifolds and the base of the carbs. If the engine increases in speed, you have an intake leak. That can also cause backfiring.
Your main jets may be too small or your air correction jets too large thus causing your engine to run lean once the carbs are off the idle jets and running on the mains.
Check the carb linkage. If one carb is opening later than the other, that too can cause backfires.
If you have a point eliminator (petronix, compu-fire etc) open up the distributor, remove the module, clean its mounting surface with steel wool and re-attach using loctite on the screw. (If that module isn't grounding properly, that too can cause backfires)
