I have a 73 Challenger with a 440. It's getting a lean back fire and we can't find the cause. We've tried everything involving the carb (even replacing it), the starter, spark plugs and wires, gas regulator and fuel pump. We're about ready to give up, any suggestions? BTW, our elevation is 6000 feet and yes, the carb is tuned for 6000 feet.

 

how about timing ?

 

No exhaust leakes ?

 

I agree with hemi head. Usually what causes backfires and pops while coming down off the rpms is some sort of leak in the exhaust system. On a four stroke single cylinder dirt bike if there is the slightest exhaust leak it will pop upon decelleration. The same theory applies to larger engines. If the backfire happens upon acceleration or just reving it up I would look in to the ignition system. poor ignition components will always cause this.