What causes a car to back fire when letting of gas?

[JIPJOBXX]

I noticed sence I have had my car that it tends to backfire a little when decelerating or when going down a hill. It does not back fire when accelerating. Could this be the timming?

[TodFitch]

Leak in the exhaust system was causing that on my car. Had to have the manifolds surfaced because they were too warped to have the gasket seal properly.

[Rodney Bullock]

Timing can also cause this to happen. My uncle's did this on purpose in the 60's because they said it sounded cool. If you back off the timing as (long as you don't have an exhaust leak) it will stop.

[martybose]

Interesting. I was conducting an experiment with my ignition system a week ago, and was driving it with initial timing at TDC, no centrifugal advance and the vacuum advance disconnected, and it was popping like crazy on deceleration. Reconnecting the vacuum advance quieted it down significantly.

Marty

[claybill]

in high school when driving about town..we had the w/wiper vac tube low under the dash. when de-cellerating or whenever we could pull it off and POP POP!!! then re-attach..

maybe you have a vacuum leak..

bill

[48Dodger]

Must in the timing like you said. a backfire through the carb or tailpipe is usally a spark in the wrong place.

Did you check your valve clearances?

48D

[JIPJOBXX]

I will get out my old trusty vacumm gage and check to see if it has a leak or not?

[JIPJOBXX]

I kind of remember an old trick to check out if you have a manifold leak and that was to spray WD-40 on the inlet ports of the manifold. This in turn causes the exhaust to blow out a cloud of oil smoke. Has anyone done this????

[TodFitch]

I kind of remember an old trick to check out if you have a manifold leak and that was to spray WD-40 on the inlet ports of the manifold. This in turn causes the exhaust to blow out a cloud of oil smoke. Has anyone done this????

I believe that this is used to find/confirm intake manifold leaks but that what happens is the mixture changes (lean from the leak going to correct or rich when the spray finds the leak) causing a noticeable change in how the engine is idling. I don't think it changes the exhaust in a definitive way.

And I don't think it helps find/confirm an exhaust leak because those will not be sucking in the spray but rather blowing out exhaust.

Re-reading your original post, is the backfire out the exhaust or out the carburetor?

[JIPJOBXX]

Exhaust and only happens when I let oft the gas or when I'm going down a hill.

[dezeldoc]

Exhaust leak, pulling cold air in on decell.

[Merle Coggins]

Backfires on deceleration usually indicates an over lean mixture. This could be caused by a vacuum leak or an idle mixture that's set too lean. I suppose timing could also do it, but I'd suspect that you'd have other issues too such as hard starting, lack of power, or running hot. A leaking exhaust may be reducing the back pressure in a cylinder allowing it to feel lean because it is breathing better. I've never heard of that causing a backfire/pop on decel, but I'll never say "that can't be it" because as soon as you do you'll be proven wrong.

Merle

[dezeldoc]

Backfires on deceleration usually indicates an over lean mixture. This could be caused by a vacuum leak or an idle mixture that's set too lean. I suppose timing could also do it, but I'd suspect that you'd have other issues too such as hard starting, lack of power, or running hot. A leaking exhaust may be reducing the back pressure in a cylinder allowing it to feel lean because it is breathing better. I've never heard of that causing a backfire/pop on decel, but I'll never say "that can't be it" because as soon as you do you'll be proven wrong.

Merle

Pull your head pipe loose and see what happens. A lean pop wil be under acceleration not decel.to rich would cause it as it would try and burn in the manifold or headpipe. Try turning off your key for about 10sec and then turn it back on at about 40mph and see what happens, it goes pop pop! or boom if ya leave it off longer!

Edited May 26, 2009 by dezeldoc

[Don Coatney]

And I don't think it helps find/confirm an exhaust leak because those will not be sucking in the spray but rather blowing out exhaust.

Tod;

I believe what is happening is air is being sucked into the hot exhaust system through a leak. This in turn will ignitie unburned fuel in the exhaust system creating a boom. Even thought the exhaust system is under pressure with the rapid flow of exhaust gases flowing past a leaky spot a slight venturi effect vacuum will be created sucking in fresh air and this fresh air will ignite the unburned fuel. The spraying of WD-40 onto the hot exhaust manifold will simply create smoke as the WD-40 evaporates faster than it can be sucked into the exhaust system. Anyway that is my story and I am sticking to it.

[Don Coatney]

Interesting. I was conducting an experiment with my ignition system a week ago, and was driving it with initial timing at TDC, no centrifugal advance and the vacuum advance disconnected, and it was popping like crazy on deceleration. Reconnecting the vacuum advance quieted it down significantly.

Marty

Marty;

I do not understand how your engine runs well the way you have your distributor set-up. As most any engine increses in speed the timing normally advances. I know you have your centrifical advance weights wire tied so they do not advance the timing. And you have your vacuum advance connect to the intake manifold not the carburetor butterfly port. On my engine the manifold vacuum drops to zero as I push the throttle open and my butterfly port vacuum increases. Now you say your engine pops on decellaration with neither mechanical nor vacuum advance. What does it do on acceleration with no mechanical nor vacuum advance in place? I am confused?

[JIPJOBXX]

Well I don't know if I solved the problem all the way but I noticed an exhaust leak just before where the tail pipe attaches to exhaust manifold. It needs to be welded up or something like that. But while checking that out I noticed one nut missing from behind the carburetor inlet manifold pipe. I must have mist this one nut a long time ago and so I installed a new one back there in the hole from hell. I took the car out and notice it was not backfiring as much (I think?). Also I adjusted the needle valve a little and its running a little better after doing that procedure. My vacuum gage shows no leaks and runs steady as it should do. So maybe the problem has been fixed and if it does still have a little miss every so often I guess that's ok as I myself at 65 have a miss every once in awhile trying to go to the bathroom. Jon:D

[martybose]

Marty;

I do not understand how your engine runs well the way you have your distributor set-up. As most any engine increses in speed the timing normally advances. I know you have your centrifical advance weights wire tied so they do not advance the timing. And you have your vacuum advance connect to the intake manifold not the carburetor butterfly port. On my engine the manifold vacuum drops to zero as I push the throttle open and my butterfly port vacuum increases. Now you say your engine pops on decellaration with neither mechanical nor vacuum advance. What does it do on acceleration with no mechanical nor vacuum advance in place? I am confused?

I have been having just a little pinging on very light acceleration, but it would stop under full or near full throttle. So I took off the vacuum advance to see what would happen. What I found is that it didn't ping under any circumstance, but it would pop back through the exhaust system if I took my foot off of the gas.

As a result of this experiment I tried limiting the amount of vacuum advance to just 10 degrees, which has improved the situation a little.

I'm slowly approaching the idea of spending way too much money for a dual channel wide range A/F gauge system, as my current suspicion is that my Carter-Weber/Edmunds combo may be going lean under light acceleration until either the secondary or the power valve gets involved. I'll need instrumentation to prove it.

Marty

[radioguy7]

I usually back fire if I have eaten too many beans!

[Powerhouse]

mine was backfiring like crazy on deceleration with the head pipes not connected to any exhaust...

[mikesinky]

my harley backfires on deceleration without the baffles, sucks fresh air in and pow pow......

[Reg Evans]

My 42 coupe was backfiring more than usual on decelleration. I discovered the fuel filter was full of junk and some must have gotten into the carb. I added two new filters and changed the carb and the back firing stopped. Time to drop and clean out the tank.

[Powerhouse]

well, mines backfiring again...

time to re-time it or something. Maybe loosened somehow...

[jim leman]

Backfiring upon deceleration is due likely to a leak between intake manifold and block. Test on cold engine, Start and spray ether at each intake port...if you notice acceleration as you spray (if you spray too much engine will stumble momentarily) there is your leak -- the intake is sucking the highly volitale fuel into the cylinder and combusting. Tighten all nuts -- you are using the correct washers (thickness) and brass nuts, right?