Emission Systems And Parts

I just replaced the smog pump on it because the old one seized up. Now whenever I rev the motor up it backfires when I let off the throttle. The carb is an aftermarket edelbrock emissions model. I'm wondering if the check valve that splits the air tube to both banks is screwed up. I need this thing to work to pass my state emissions test. Any ideas?

P.S. It's a 350 w/ a 4 barrel
No it does not have an exhaust leak, the timing is good, and the spark plugs are a couple months old. I've been running this truck with no smog pump for a year with no backfiring. Now with the new pump connected it starts backfiring.