Why am I backfiring.

[zach78k5]

I have a brand new holley projection 650cfm 2D system and I recently installed a new distributor. My old one was ESC and I replaced with HEI distributor. When I let off the pedal during harder accleration, I always backfire. Also, sometimes when the engine is cold it will backfire much more even if i am giving a little throttle. On a similar note, sometimes when the truck is cold, it feels like it is running on about 7 cylinders but once it warms up all is fine. I feel like these problems are all from the same source. Can a muffler or catalytic converter cause the backfiring at all? It did the backfiring after hard accel before the holley projection was there, the other problems seem to be more recent though.

 

[trailblazer86]

A hole in the exhaust system could cause a backfire.
If you have a hole in the exhaust it will allow fresh air into the system, and the fresh air will allow the unburnt gas that is still hot enough to ignite to cause the pop.
Leaky headers almost always give you that exhaust POP when you let off the gas during a hard run.
Coasting while in gear will also cause the pop somtimes.