Backfire through Carb

Hi All,

I have a 69 Valiant with a 273. Engine is backfiring through the carb when I accelerate and looses power. I've replaced the accelerator pump, checked the spark plugs, wires, timing and also tried a new distributor. Just can't get rid of it. Any other ideas on what might cause this. Car was running great not too long ago.

Thanks all for any help!