The Stovebolt Site Forums The Shop Area Engine Shop 235 backfire on deceleration

Starting to get frustrated on this tuneup. I have a 54 chevy with a 235. it has holley 390, split headers, pertronix ignition. i have been trying to tune it correctly for awhile. I rejetted carb and changed power valve to what is recommended from clifford performance. My problem is while driving at higher rpms, when i let off the gas, i get lots of popping and backfire through the exhaust. that is my only problem. the throttle response is good, no hesitation or backfiring on acceleration. I have timed the engine from 0 degrees all the way up to 20 degrees advanced. When I retarded the timing, it got worse. When I advanced it, still backfire issue. the spark plugs look good, not to lean or too rich. Only backfires from high rpm deceleration. Also it is not initially when i let off the pedal, its almost when the rpms get back down to about 2k. Any help would be great, I am not sure what it could be. Thanks!

Try enriching the idle mixture screws a little. This can be caused by a lean idle mixture. Vacuum leaks can also cause it.

Did your tune up include setting your valve lash?

It may not be your problem but its worth checking.

I had the popping on deceleration once. It ended up being a bent push rod throwing off my valves on one cylinder.

Good luck

I have tried adjusting the mixture screws, more popping the leaner the mixture, I am 2 1/2 out now, if I go more out, there is a hesitation and still the backfire too.

I have not checked the valve lash, a little nervous about adjusting them. I dont want to screw anything up haha.

Make sure that there are no exhaust leaks, anywhere. On decel any fresh air that can be drawn into the exhaust system will ignite with hot unburned fuel and explode, causing popping and backfire.

Have you checked your vacuum advance?

I'm going to cast my vote with Joe.
The leaks probably are in the first 18 inches of exhaust tract.
Make a little smoke by feeding it a little oil/gas mix or diesel through the carb and watch for leaks near the manifold joints.
Plug the exhaust outlet with it running while a helper watches and listens around the manifold.
EXHAUST LEAK

how far should I advance my timing with the pertronix and 4 bbl carb? also, when I adjust my timing, I am seeing the timing mark about 4 different places when i twist my distributor... which mark should i go with for correct timing? thanks

there is only one timing mark, the little bb imbedded in the flywheel. Take the pertronix out, at least until you get this sorted out, but timing is the same regardless of what ignition system you use. You will have to adust timing whenever you switch from points to pertronix, and vice versa. I was extremely underwhelmed with pertronx myself, the old truck never ever failed me in 28 years until I UPGRADED the ignition, I could work on points for the rest of my life and still not have gone through as much misery as that one day.

My experience with that problem in various vehicles has been it is often caused by a lean condition.