The Stovebolt Site Forums The Shop Area Engine Shop Deceleration noise

My '54 3600 - engine with 1000 miles since rebuild has started backfiring through the exhaust upon deceleration (ie; let off the gas after acceleration or when headed down a hill).
What's the likely culprit - valves ?
Any help is appreciated.
Thanks
Kevin

Mainer54,

I have been trying for two years to eliminate the same thing. Check your valve lash, ignition timing, and carburetor settings. I lessened the backfiring after sealing a few leaks in the exhaust system (dual carbs and dual exhausts). One thing I have not checked is a leaking carburetor (there is an internal and an external leak - maybe due to a bad float setting).

I have been told that unburned gas in the exhaust and an air leak providing oxygen to the exhaust can result in an ideal condition for backfiring (backfiring out the exhaust, not out of the carburetor).

Good luck,
Tim

I would be checkin' that heat riser valve.
If that sucker gets stuck in the closed position it can cause all the problems you listed.

I did my own exhaust a few months back and had the same problem until I ran mostly straight (couple of curves) pipes all the way to the back bumper (headers w/ dual exhaust).
Did you do exhaust work? Is everything routed the same as before the rebuild?

to clear something up when you hear a pop out of the tail pipe it is called an after fire in which unburned fuel (Hydro Carbons) gets pushed out the exhaust with oxygen and due to high exhaust temps it ignites in the exhaust which causes an after fire. A true backfire come from the intake when the air fuel mixture ignites in the intake manifold and comes out the intake. Backfiring isnt necisarily bad. Most older vehicles do it when you are cruising or give it lots of gas and let go of the throttle. New cars do it to if you take off the resinator and muffler. So in my experienc ei wouldnt say it is bad but obviesly doesnt sound pleasant. Theres my 2 pennies!

Shorter or chopped exhaust systems have a tendancy to afterfire. If the system is full-length then check engine tune and exhaust system clamps/flanges.

I knew a guy that had a F-100 with a granny 4 speed and a 351M. He had glasspacks and a serious knack for messing with people. He would drive by the gas station I worked at in college late at night. He would get in front of the station, cut the ignition off then cut it back on. This would produce two shotgun blasts that would send the customers diving for the floor, doughnuts and coffee flying about.

The cops actually came flying up in the parking lot just after he left. Came in the store and talked to me:

"we had a report of gunshots fired"
"actually, it was a westbound **** ."

This happened about two more times. The forth time the cop comes in and says:

"green truck?"
"yup, F-100 in regress"

My '52 216 started that when I installed a new exhaust system. New system was more straight-through than the old one that was hush quiet. I've tried everything, carb settings, timing, valves, heat riser...just learn to live with it I guess.

Lean idle mixture or vacuum leaks can cause
a backrap what we used to call it. In fact
in my younger days we used to screw the idle
jets way in. It sounded real cool.

Thanks for all the help everybody, I think I'll try giving it a thorough tune-up starting with the valve lash, themn timing then carb tweaking.
The exhaust is stock and in good shape and the noise has developed recently, so a tune-up or looking for a leak sounds like the best bet.

When my truck starts making those popping sounds, I know it's time to retorque the header bolts.