My 1981 F-150 Ranger with a 351W engine and a 2100 Motorcraft carburetor backfires when I first start it after it warms up. In the morning when the engine is cold, it starts right up without backfiring, but when I go to the store and try to start it, it backfires, then starts. What do I need to do to fix this?

 

possibly the power valve. i dont know too much about fords factory carbs.