New motor backfiring through carb

Got the new motor ready to fire up tonight. Got fuel primed to the pump. Got fuel pressure at the carburetor. Carburetor ran great on the dyno and the engine ran great on the dyno. Went to start it tonight and it would crank and crank and crank then the starter would slow down and then a backfire out of the carburetor. I worked on the car for about 5 hours tonight getting ready to fire it. This happened to me three times and finally I through in the towel at 10 and went home. Any clue what could be causing this?