miss
Race Director
Joined: Jan 2002
Posts: 15,353
Likes: 2,726
From: Close to DC
C2 of the Year Finalist - Modified 2020
Most likley it's in the ignition system. Recheck your timing, initial, centrifigal, and vacuum, point set. Others here may give you more details. 
Dennis
Dennis
Advanced
Joined: Jan 2011
Posts: 65
Likes: 12
From: Arkansas
I agree sounds like and ignition problem. When using your timing light ensure that there is a smooth increase in timing as engine rpm increases. There may be a stuck centrifugal weight or the vacuum advance may be sticking. Another check would be the distributer cap and rotor if there are carbon trails that could be the problem.
Pro
Joined: Jul 2008
Posts: 622
Likes: 24
From: Clermont Florida
If it is a constant backfire, that increases in loudness when you accelerate, you have wiped an exhaust lobe on your camshaft. to locate the cylinder, remove one plug wire at a time from the cap, and jazz the throttle. The popping will reduce considerably when you hit the right cylinder. A lean condition will cause a single backfire combined with a lag, or hesitation. Ignition crossfire would be intermittent, and not continuous.
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Melting Slicks
Joined: Oct 2004
Posts: 3,345
Likes: 87
From: Northern MN
St. Jude Donor '13-'14-'15-'16-'17
I had a couple of broken valve springs in a 427 that I had in my 66 Chevelle. I only had a miss in the engine. Never had the backfire. I didn't think about it at the time but both that broke must have been on the exhaust side. I went ahead and replaced all 16 anyway. At the time they had designed the new double spring unit with the dampener to cure the problem. I believe in 67 those springs had become standard for the 435's and L-88's. JohnZ will know that.
Thread Starter
Racer
Joined: Nov 2003
Posts: 353
Likes: 1
From: Sugar Land, TX
Thank you for your help. Turned out to be spark plug wires. However, I did check for timing chain jump and bent or broken valve train parts. Engine is running strong and ready for next road trip and repair lol.
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Army
Backfiring in the tailpipe (often called 'afterfire') is raw gas detonating in a hot tailpipe. I had it sporadically back when I had faulty needle valves in the carbs which fouled my plugs horribly. However, all of the above suggestions apply -- although I haven't seen in a blown power valve in Holley carbs in many a year with the later versions...