396 bbc back fire through carb

iv got a old school 780 holley on it it was back firing through the carb when i would rev it up or when you would be going down the high way giving it gas going through the gears or if you would stomp on it it would pop and it wouldnt go very good so i advanced the timing thinking it was retarded to much but it didnt go away and i replaced the power valve still same thing. i switched carberators to a 600 double pumper just to see if it still did it. it still did. no mater how little or how much timing i give it. it still back fires and doesnt drive good at all but suprizingly it idles pretty good. if i advance to much i pops out the exaust. the engine only has 25 miles on it. it has 550 hp 650 ft of torque.


any help would greatly be appriciated im out of ideas-