Back firing on decelleration

[Hedgehog]

Stats
1982 rebuilt AMC 360

Stock 2150 Carb, professionally rebuilt not long ago

Dui HEI distributor set at 12* -the vacuum was disconnected, used stock gear Edit: stock springs. The vacuum was disconnected while setting the distributor as is standard operation.

Inside the rails headers (look to be headmans)

Cats

Flowmaster mufflers

Stock air pump

Problem - The darned thing back pops and takes on with an occasional hard fire on deceleration. It is worse if I've just gotten on it, as in some Lexus driving smart *** tries to cut the CJ off pulling away from a stop light and traffic suddenly slows up. It also happens less so during more mundane times like adjusting speed on the interstate.

I've talked to the carburetor guy. He said the timing was good and the carb seemed good, he had no solution except to say that a leak in the exhaust would do it. The exhaust system looks like new. Frankly it is driving me a little nuts to go rattling and banging along down the road. Disclaimer: it's not really that bad, but the occasional back firing is disconcerting.

So, I'm bringing this to the board in hops that someone might know what to do.

 

[Torxhead]

So why was the vacumn advance disconnected and have the advance springs ever been changed. I'm sure you have no vacumn leaks and the spark plug deposits look good. I think you should make sure that the mechanical advance is not sticking also.

 

[Hedgehog]

When ever setting advance is mentioned someone always asks if the vacuum advance was removed and plugged while the distributor was being set. The DUI dist. is relatively new and its always done it. the carb shop checked the advance when looking with the jeep. Stock advance springs are being used.

 

[Posi]

Have you checked all the bolts on the exhaust?

 

[Hedgehog]

The header bolts have been checked. The exhaust is welded except for the collector gasket bolts. I did get new header collector gaskets, it would be easy enough to install them.

 

[Torxhead]

From what I understand the D.U.I. is a very adjustable distributor and the advance can be adjusted also. Just to try something out, how about backing off the initial advance which would in turn change the maximum advance and see how that works. I do think 12 Deg. is a bit on the high side though. Hopefully you can get an ignition expert to chime in.

 

[Lumpy Grits]

For c'its and giggles-Back the timing off 3* and see what happens.
You run'n reg or prem gas.
Must you run the airpump?
LG

 

[oss]

When checking the timing does the timing mark bounce around(as in higher and lower degrees)? Could be timing chain.

 

[hole]

My 8 has done it the entire time I've owned her and was worse when it still had the stock AMC 258 i6 / 4.2l head and stock exhaust, actually much worse. Now, with the 4.0 head and 2 1/2" exhaust it only does it on hard deceleration's and not bad at all. Sorry Hedgehog, not much help here

 

[Fastatv]

An exhaust leak can def cause it to backfire, but your carb may be a bit lean as well. Have you tried turning out the a/f adjustment screws an additional turn and then drive. Might give that a shot.

 

[Hedgehog]

I will back off a few degrees, but if I remember correctly timing had little effect on the popping issues. Many folks point to exhaust leaks. The new header collector gaskets will also go on. I replaced the manifold gasket last fall and everything seems tight there.

oss - The timing mark is steady, no bouncing, but a good suggestion.

Fastatv - It actually seems a bit rich. Ignorance on my part, what is an a/f screw?

 

[Torxhead]

Another possibility could be carbon buildup in the combustion chambers, caused by slow driving. Carbon deposits can cause a misfire as it gets hot. A fuel additive like a
BG44K or the less expensive Chevron Techron in the gas tank will help. A run down the highway is one more method to clean it out.

 

[Fastatv]

I will back off a few degrees, but if I remember correctly timing had little effect on the popping issues. Many folks point to exhaust leaks. The new header collector gaskets will also go on. I replaced the manifold gasket last fall and everything seems tight there.

oss - The timing mark is steady, no bouncing, but a good suggestion.

Fastatv - It actually seems a bit rich. Ignorance on my part, what is an a/f screw?

Hey Hedge, there are two screws in the base of the carb, one on each side, front of carb and at the very bottom. They should require a "flat blade type" screw driver. Turn each one a complete turn counterclockwise. This will richen up the idle circuit and may reduce the popping when letting off the throttle. Cost nothing...worth a shot. If no change, then place the screws back where they were....

Torx has a great point as well. Carbon build up, when hot, can cause detonation. Fuel additive isn't expensive....worth a shot as well.

 

[Lumpy Grits]

Again-IF you don't need to run the air-pump. Ditch it.
What color are your plugs-They will tell you the story of your fuel mix.
Did this back-firing just start?
LG

 

[Hedgehog]

No, as time passes its beginning to bug me a little more. To pass inspection I have to run the pump. If I lived out of the metro area it could be removed.

 

[Lumpy Grits]

OK-understand the SMOG deal-TRUST ME, I really understand the stupid SMOG deal on a 40 yr 'car'.
Same issue here in PRK with my '85.
Make sure the air-pump fittings are not leaking to the exhaust.

LG

 

[Hedgehog]

My spiders are welded on, the individual manifold valves are new and the hoses seem tight, but I've always wondered about the old second hand routing valve though. They do make replacements, I should look into that.

Eventually I would like to install a Howell Fuel Injection kit that will allow me to legally remove the pump.

 

[jammer1]

In the 80's I had a CJ7 with a AMC 304 and the air pump. On hard deceleration I would get a back fire at times. If I remember right I disconnected the vacuum line (on the diverter valve) and that stopped it . The diverter valve went bad on mine. If you remove the rubber hose (the air flows through) that also stopped the backfire (you just heard the pulsing of the air pump). Theirs a check valve in the metal line to reduce the exhaust noise (and save the diverter from heat). You can try that to see if it's the air pump. If the vacuum hose disconnected works you can drive it unconnected and reconnect for inspection. Hope this helps.

 

[Hedgehog]

Interesting side discovery. I dropped in at Willys Works in an attempt to buy a NOS air injection system diverter valve thinking that mine could be leaking. Rob had sold the valve he had so he pulled out his vacuum gauge to check the used valves he had. We checked 6 valves including 3 on vehicles in the yard. Not one would hold vacuum, when we checked mine, by golly it held vacuum. So much for that being the cause of the popping problem. No super easy fix available there.

 

[Lumpy Grits]

Try back'n off the time'n some. 2-3 degrees should do it.
Also-make sure the 2 idle mixture screws are backed out the exact same.
Sometimes, those screws will 'creep' some, and change the setting.
Unequal settings here, can also cause the popping you speak of.
Good luck-
LG