Originally Posted by Asmodeus

Often times during a header swap, people will crack spark plugs.

Originally Posted by Fire-formula 93

Mine was backfiring after header install also. Definitely check the plug wires. I have MSD wires and they are very difficult to snap on and even then it is not a very assuring "snap on". I say pull each plug wire and reinstall them paying very close attention that they are being fully connected. I could have sworn mine were, and they were not. On my wires, I learned that even though I was pushing as hard as I could and were as far as they could go, a couple still were not connected properly. I also had this problem with the coil pack connections too.

Did you install the plugs before or after the headers were in? If you installed them before the headers, you may have hit a plug and cracked the porcelain also. I installed my plugs after the headers were in to avoid this.