So recently did the exhaust and retired the beat manifolds and cross over and went to long tube headers and full 2 1/2 duals with 2 chamber mufflers. Since doing it been having backfiring everytime i let off the throttle while driving. At first it was low and a few small pops like you would get down shifting but recently noticed if i do not very slowly ease off the throttle i tend to get a backfire that sounds like a shotgun blast. Checked for vac leaks and found none checked the carb to make sure it had no problems still cannot figure out why timing is 3 degrees advanced has been since i installed the motor. I've wanted to contribute it to the headers since never did it with stock exhaust but at a loss as to the reason of it.

 

I probably should have said backfire is exhaust been blowing out header collector gaskets like stupid from it has not backfired from carb at all which is what confused me. Ran the car around with no hood and had a go pro mounted next to the air cleaner and backfire comes from under the car.

 

Possibly running too rich. You have residual fuel in the exhaust manifold area. Your detonation is incomplete w/in the combustion chambers. The hot exhaust manifold is igniting the residual fuel. Check your spark plugs to see if they are burning rich i.e. black as coal (rich), white (lean), sandstone (14.7 = proper A/F mixture). Did you establish a vacuum reading for your A/F mixture screws? With a vacuum gauge you need to establish the highest vacuum you can achieve. If you don't have a vacuum gauge, adjust each A/F mixture screw to the highest RPM you can attain.

 

I figured as much Skribble, I'm guessing you had seen my post from early yesterday before I deleted it. For some reason when I went to post, the carb backfire page came up, sorry about that. Since then Vintage Chief has covered it thoroughly. In any case here is the page I intended to post in the first place. Its a short read that backs up what he said.

https://itstillruns.com/troubleshoot...e-5548627.html

 

That link talks about a lean A/F mixture backfire - which can also occur; but, generally I've seen lean A/F mixture backfires occur via the carb/intake manifold as opposed to the exhaust. Again, I'd check the plugs just to be certain and see if you're running too rich. The other thing to note is make sure each spark plug has the correct gap. Sometimes it's easy to forget and you've inserted one plug with practically no gap whatsoever. It really can take just one plug with a bent (crushed) spark plug wire electrode to cause a backfire - in particular when decelerating. Or, the possibility of a damaged/cracked/weak/broken electrode. Check each spark plug thoroughly. Spark plugs have been known to crack. If you have one broken/cracked/weak spark plug you'll obviously get very poor detonation in that cylinder. While accelerating the car won't be very peppy in this condition but can certainly backfire on deceleration. Three other things: (1) Separate spark plug wires from each other as much as possible so they're not all laying on top each other generating cross-firing; (2) Make sure your A/F mixture screws are set to the highest achievable vacuum; and, (3) ensure your timing is correct.