carb backfiring

Sounds like it's lean if it backfires when you go to the floor from a idle. May not be enough pump shot, or the idle mixture screws are out of adjustment,. The engine can be made richer by turning the idle mixture screws out but go 1/4 to 1/2 turn at a time. As this increases the fuel a good bit at idle. What size jets are in the carb?