I bought a truck with a 454 in it. The guy said timing was off. I have had it sitting in the driveway for a month now. I moved the timing a bit, adjusted the carb, but it kept having this intermittant miss and backfire. Tonight I was just going to pull it out with plans to re-cam it and check things out. Then as I started disconnecting the battery, it dawned on me I didn't check the plug wires for firing order. 1-8-4-3-6-7... wait... 7? That one should go to 5. So I switch the plug wires and now the backfire is gone. The guy who put the wires on wasn't too smart... but I'm not too much smarter for overlooking that the past month.