popping from exhaust on deceleration

[Bill Monk]

First, let me say I appreciate everybody that has taken the time to give advise to me on this site. It has proven to be an invaluable tool to diagnose problems so thanks!

Now to the problem: Engine has a slight hesitation at low throttle but runs like a scalded dog when you step on it. When I take my foot off the throttle, there is a random popping from the exhaust. It isn't a loud bang but a pop pop and it is random. It doesn't seem to do it when decelerating from a light load.
I'm pretty sure it is not a vacuum leak and the car has new plugs. One thing I noticed when trying to get it timed in is that the vacuum advance on the distributor does not appear to be working. I have it plugged to manifold vacuum and I saw no change with it plugged off v/s plugged in. I rather doubt that has anything to do with the popping but just throwing that out there. Does this sound like a timing thing or what?

 

[the mississippi madman]

Maybe to lean

 

[miller]

One thing I noticed when trying to get it timed in is that the vacuum advance on the distributor does not apprear to be working.

A quick way to check if the distributor advance is working, pull the cap, and see if the rotor will move in one direction a little, then move back into place.
Doesn't matter what kind of distributor, the advance 'plate', that the points mount to, rotates a number of degrees. That's how the advance works. Does it move?
If not...could be a couple things, though the usual problem is it's dry, and hung up. If you pull the rotor itself, look at the top of the shaft, where it fits. Should be a small felt 'oiler' just inside the top of the shaft. The main shaft on the distributor, is a smaller diameter, at the top, with part of the advance plate that fits onto that smaller end. Needs to be able to rotate, and spring back into place.
The oiler felt should get 'serviced' now, and then, by putting a few drops of oil on it. Let the oil soak in, then wipe off the excess. Oil in it, works it's way to the space on the shaft, that rotates...lubing it. If it goes dry, yeah, can rust and lock down.
Worst case, is you have to tear down the distributor, to get in there, clean it up, to get it to work.

 

[Bill Monk]

A quick way to check if the distributor advance is working, pull the cap, and see if the rotor will move in one direction a little, then move back into place.
Doesn't matter what kind of distributor, the advance 'plate', that the points mount to, rotates a number of degrees. That's how the advance works. Does it move?
If not...could be a couple things, though the usual problem is it's dry, and hung up. If you pull the rotor itself, look at the top of the shaft, where it fits. Should be a small felt 'oiler' just inside the top of the shaft. The main shaft on the distributor, is a smaller diameter, at the top, with part of the advance plate that fits onto that smaller end. Needs to be able to rotate, and spring back into place.
The oiler felt should get 'serviced' now, and then, by putting a few drops of oil on it. Let the oil soak in, then wipe off the excess. Oil in it, works it's way to the space on the shaft, that rotates...lubing it. If it goes dry, yeah, can rust and lock down.
Worst case, is you have to tear down the distributor, to get in there, clean it up, to get it to work.

I'll check it out but I don't think that's the culprit. I have mechanical advance at higher rpms.

 

[moes]

do you have a free flowing exhaust? Any time the exhaust is “opened up” the condition of deceleration pops is much more noticeable and problematic. An open exhaust allows additional air to be “sucked up the pipe” during closed throttle and any unburned fuels, then ignite and “snap/crackle/pop”

 

[khryslerkid]

do you have a free flowing exhaust? Any time the exhaust is “opened up” the condition of deceleration pops is much more noticeable and problematic. An open exhaust allows additional air to be “sucked up the pipe” during closed throttle and any unburned fuels, then ignite and “snap/crackle/pop”

I'll agree to the open exhaust and I believe there is more oxygen in the system than in a conventional factory setup.

I removed the inside honey comb from a catalyst converter once. It was breaking up and clogging the pipe. It had to have one on there for inspections so I just gutted it out and reinstalled. That was a mistake, it would build up unburnt fuel and backfire like a 12 guage shotgun on deceleration!

 

[Bill Monk]

do you have a free flowing exhaust? Any time the exhaust is “opened up” the condition of deceleration pops is much more noticeable and problematic. An open exhaust allows additional air to be “sucked up the pipe” during closed throttle and any unburned fuels, then ignite and “snap/crackle/pop”

Stock manifolds, 2 1/4" pipes with a crossover through hemi mufflers.

 

[Fran Blacker]

Stock manifolds, 2 1/4" pipes with a crossover through hemi mufflers.

If you want to test the diaphragm put a hose and "suck" through it and stick your tongue over the hole if it doesn't hold a vacuum it's bad. You maybe able to see plate move with cap off.

 

[Ranger16]

do you have a free flowing exhaust? Any time the exhaust is “opened up” the condition of deceleration pops is much more noticeable and problematic. An open exhaust allows additional air to be “sucked up the pipe” during closed throttle and any unburned fuels, then ignite and “snap/crackle/pop”

That does sound a lot like what you see on NASCAR races.

 

[RJRENTON]

Depending on the year of the car and if it has any of the original emission control equipment, some cars (usually a manual trans) engine transmission combinations had a deceleration control device that advanced the vacuum to the distributor. Later models had a Electric retard system as part of the distributor 's vacuum advance that retarded the spark at closed throttle to prevent an over rich condition and "popping" in the exhaust.
The other contributors suggestions should be considered. Will or is the popping exhaust harmful? Possibly. Check the condition of the spark plugs to see if the engine is too lean. It sounds like you may have a vacuum leak causing an off idle stumble.
Bob Renton

 

[383man]

do you have a free flowing exhaust? Any time the exhaust is “opened up” the condition of deceleration pops is much more noticeable and problematic. An open exhaust allows additional air to be “sucked up the pipe” during closed throttle and any unburned fuels, then ignite and “snap/crackle/pop”

I see this many times. A small exh leak at the manifold/header can suck in air on decell and cause the poping/backfire in the exh. Just like cars with air injection pumps as they dump the air pumped into the exh on decell so they dont backfire through the exh. If the diverter/dump valve goes bad and they dont dump the air to atmosphere on decell they will backfire through the exh. Ron

 

[Bill Monk]

Problem solved, I had removed the distributor cap and I guess when I put it back on, I didn't center the cap and bent the spring on the top of the rotor....mom was right, I am an idiot!

 

[khryslerkid]

Problem solved, I had removed the distributor cap and I guess when I put it back on, I didn't center the cap and bent the spring on the top of the rotor....mom was right, I am an idiot!

Thanks for the update