The Stovebolt Site Forums The Shop Area Engine Shop Backfire or ....

Although my truck has done this for almost since the time I put the motor in, I became really annoyed with this problem recently (probably the hot weather making me irritable). Soon after installing my new motor and exhaust system (around 2 and a half years ago), the truck began to "backfire" on deceleration. I am not sure what the problem is. The truck runs great, I have timed it the best that it possibly could be and it doesn't miss a beat when you hit the gas. Every time you let go of the gas and it begins to decelerate it randomly makes loud popping noises out of the exhaust. The specs are as follows 350, 30 over, original 66 heads off the 327, RV cam, 4 barrel Edelbrock, dual-exhaust with 22" glass packs. Is it possible that the glass packs are damaged? I also am considering that there is backpressure on the engine and it is pulling in cooler exhaust and causing backfire on deceleration. Any ideas? Thanks

You need some real mufflers. Glasspacks are straight-through design, and it's not uncommon to get a backfire on deceleration, particularly if the mixture is leaning out a little as the throttle is closed on a coasting situation.

I like to use Cadillac mufflers from the mid-70's. The inlet and outlet pipes are both 2 1/2" diameter, and one muffler will handle the volume requirements of a 500 cubic inch engine. Dual pipes with those mufflers installed have virtually no back pressure, and they're QUIET!
Jerry

Sounds like you are running slightly rich on the mix and you are getting unburned fuel through the system that is igniting in the exhaust system. Fairly common with glasspacks as they tend to run a little hotter than a standard muffler. Try backing off on the fuel mix a little and see what happens!
Laterz,
Gator
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Dual exhaust...glass packs...had the same issue....it was fuel too lean in our case my uncle backed out the Rod B screw an 1/8 of a turn and that's all it took. Them old guys know! T/C Steve

Ok, good to know, I will mess with it and see what happens. Thanks all.

Make sure you don't have an exhaust leak at the manifolds. I use Flowmasters, no packing to set off unburnt exhaust, and they sound great,
Scott

Already checked for leaks. Haven't adjusted carb yet though.

Make shure there are no exhaust leaks or vacuum leaks. Richen up the idle mixture adjustments a little & it will cure it. We used to lean the idle way down many years ago to make it do that because it sounded cool.

Running a lean idle mixture can cause exhaust pop. Late timing can cause it also. Try tinkering with your idle mixture and bump up your timing a little. Exhaust has nothing to do with popping, except that is heard more with loud mufflers. see what happen.