Carburetor backfire??

I recently rebuilt the top end of my jeep: new pistons, rings, valves, reworked head. It runs but the carburetor hesitates upon acceleration and backfires often with flames. I have set the valves numerous times only with the same results. What would cause flames to come out of the carburetor?
It seems to me that for flames to come out of it, the plugs would have to spark while the intake valves are open, and that doesnt seem like it could happen. I know it could also be the intake valves not seating properly but i had the head reworked and the valves ground.

Its normal as long as it only happens when your engine is cold.