Originally Posted by dgcantrellsr
If all you changed was the mufflers and car was not popping before, it ain't the mufflers. Mufflers may slow it down a fuzz, but they won't cause backfire.
What else did you do? Any other adjustments,timing , valves,change plugs, etc.?
5&7 wires crossed?
Sounds like spark timing to me, too. What type of Ignition, MSD? How were the mufflers mounted, WELDED? I recomend pulling your engine and getting it DYNO tested with your complete ignition system.