Mild backfire through mufflers

[furyus2]

As the title says, when my small block is cold, I hear a couple light backfires through the muffler. Goes away when warm. Has a new distributor from Pertronix. Has a 600 Holley . Charges well,starts fine, runs great when warm. Mild 484 lift cam. Any ideas?

 

[fishy68]

With a performance cam nothing will run great cold. If the carb mixture is adjusted correctly with the engine hot it's most likely a lean mixture misfire. Some spark plugs seem to be more susceptible to a lean misfire than others. You might try a fresh set of plugs.

 

[furyus2]

If I try a fresh set of plugs, should I go hotter,colder,to stay the same?

 

[Opm68]

I had a similar problem. The first time I discovered that the secondary throttle on my new Holley was adjusted too far open. Adjusting the throttle stop by turning the screw about 1/2 turn cow solved the problem. Years later I encountered the same problem this time the source was a leaking intake manifold gasket. Look for a vacuum leak. Technically the backfire in the muffler is called afterburner.

 

[supertruck]

you might also verify you don't have a blown power valve in the carb. that can cause a backfire also.

 

[fishy68]

If I try a fresh set of plugs, should I go hotter,colder,to stay the same?

If their correct keep the same.

 

[fishy68]

you might also verify you don't have a blown power valve in the carb. that can cause a backfire also.

You rarely see blown power valves anymore because in the early 90's Holley implemented an anti backfire valve in the baseplate that closes in case of a backfire. Unless the carb is really old it's unlikely the PV is blown. And furthermore I don't see how a blown PV could cause a backfire because a blown PV lets the engine run real rich. It'd run like dog crap but an overly rich mixture is not likely to backfire.

 

[Inertia]

You rarely see blown power valves anymore because in the early 90's Holley implemented an anti backfire valve in the baseplate that closes in case of a backfire. Unless the carb is really old it's unlikely the PV is blown. And furthermore I don't see how a blown PV could cause a backfire because a blown PV lets the engine run real rich. It'd run like dog crap but an overly rich mixture is not likely to backfire.

^^^^^^^ Xs2..

Do you have an exhaust, header, manifold, or muffler leak.??

 

[furyus2]

Headers, no leaks. Thanks, I will check everyones suggestions.

 

[Bad Sport]

Do you have an exhaust, header, manifold, or muffler leak.??

That was my first thought.

OP, are you revving the engine slightly and letting off when you hear the popping? When you hear it, are you revving, or do you hear it when you let off?

 

[supertruck]

You rarely see blown power valves anymore because in the early 90's Holley implemented an anti backfire valve in the baseplate that closes in case of a backfire. Unless the carb is really old it's unlikely the PV is blown. And furthermore I don't see how a blown PV could cause a backfire because a blown PV lets the engine run real rich. It'd run like dog crap but an overly rich mixture is not likely to backfire.

I suggested a blown PV because I wasn't sure if the backfire was through the carb or the exhaust. a blown PV will cause a backfire through the exhaust because of the extreme rich condition.

 

[furyus2]

Seems to happen when I start to accelerate moderately from a low rpm, like when in 2nd gear after making a turn. I think the peculiar thing, is it goes away after warm up.

 

[Bad Sport]

Seems to happen when I start to accelerate moderately from a low rpm, like when in 2nd gear after making a turn. I think the peculiar thing, is it goes away after warm up.

That's pretty indicative of an exhaust leak. Is it something you can feel?!? Or you just hear it?

 

[TrailBeast]

Seems to happen when I start to accelerate moderately from a low rpm, like when in 2nd gear after making a turn. I think the peculiar thing, is it goes away after warm up.

Are you pretty sure it's not a valve adjusted too tight? (if you have adjustables)
When it warms up and everything expands it could go away because the valve is now closing all the way.

On another note I had one doing that not long ago and no matter what I did it wouldn't go away, so I pulled the idle mixture screws, shot some WD40 into the ports and blew through it with my compressor.
Readjusted the idle mixture and it was gone.

Lean idle mixture was causing it because of a partially clogged idle circuit.

 

[furyus2]

Are you pretty sure it's not a valve adjusted too tight? (if you have adjustables)
When it warms up and everything expands it could go away because the valve is now closing all the way.

On another note I had one doing that not long ago and no matter what I did it wouldn't go away, so I pulled the idle mixture screws, shot some WD40 into the ports and blew through it with my compressor.
Readjusted the idle mixture and it was gone.

Lean idle mixture was causing it because of a partially clogged idle circuit.

The rockers are adjustable, but I haven't changed the settings on them. I will try the mixture screws first. There's no exhaust leaks that I have heard, but I'm gonna go through all these sussestions.

 

[m880]

Your choke is causing it to run to rich during warm up and the excess fuel will ignite and cause exhaust backfire.
Back off on it a little bit and see what happens.

 

[RustyRatRod]

Backfiring at idle or low RPM is caused by an overly rich mixture. Whereas backfiring at high RPM is caused by an overly lean mixture.

A carbureted engine is always overly rich until it gets up to operating temperature. A carburetor must have some heat in it in order to atomize fuel right.

This is why blocking the exhaust cross over on anything but an all out race car, or something that never sees very cold weather is usually a bad idea.

More than likely if you try to tune it out, you will be too lean at operating temperature. As long as it stops when it warms up, I wouldn't worry too much about it.

 

[skizzo]

If it runs great warm....
Call me crazy, but since you don't tune to run below normal operating temp why change parts/get finicky on cold operation?
Cars run weird cold. Try running choke less at 10,100ft with 7deg. ambient temps lol, gets real wonky..

 

[TrailBeast]

Backfiring at idle or low RPM is cause by an overly rich mixture. Whereas backfiring at high RPM is caused by an overly lean mixture.

A carbureted engine is always overly rich until it gets up to operating temperature. A carburetor must have some heat in it in order to atomize fuel right.

This is why blocking the exhaust cross over on anything but an all out race car, or something that never sees very cold weather is usually a bad idea.

More than likely if you try to tune it out, you will be too lean at operating temperature. As long as it stops when it warms up, I wouldn't worry too much about it.

Well if he's sure it isn't valve not able to close all the way I would have to agree with this, and the others along this line.

 

[justinp61]

A carbureted engine is always overly rich until it gets up to operating temperature. A carburetor must have some heat in it in order to atomize fuel right.

I'll agree on a carburetor with a choke, without a choke and it'll be lean until it warms up. I run a 950HP and the afr's at idle cold are in the 16-17 range, after it warms up 13.8-14.1.

If you non choked carbureted engine idles great cold it's pig rich warm.

 

[RustyRatRod]

I'll agree on a carburetor with a choke, without a choke and it'll be lean until it warms up. I run a 950HP and the afr's at idle cold are in the 16-17 range, after it warms up 13.8-14.1.

If you non choked carbureted engine idles great cold it's pig rich warm.

Nope, that's not the case. Here is why. The carburetor must have heat in order to atomize fuel completely. Until it does, the overall condition is rich.

Now, RIGHT AT startup when one is cold, you are correct. They need that extra dump of fuel the choke sucks in on initial startup, but once running a cold engine is rich, even without a choke.

 

[justinp61]

I guess mine and a bunch more afr meters are wrong. LOL

 

[skizzo]

I guess mine and a bunch more afr meters are wrong. LOL

Usually those don't operate accurately below 350deg., theyll cycle, idk which probe/meter you run.

 

[justinp61]

LM-2, it pre heats the probe.

 

[nm9stheham]

AFR sensors aren't really measuring AFR; they are measuring free oxygen content and are giving AFR based on assumptions about the fuel and the completeness of the combustion process and integrity of the system. If there are wet fuel drops not being burned, it won't use up as much oxygen, and the oxygen content will be high... looking lean on the AFR meter. The actual mixture is different than the meter shows in this condition. The heater only makes the O2 sensor more accurate at measuring free oxygen content; it does not correct for unburned fuel, or air leaks, or wild overlap issues, or any other issues that put excess oxygen into the exhaust.

From an article on wideband sensors: "For example, if a miss-fire occurs and unburnt droplets of fuel go through the motor, then the sensor will read lean as it will not detect the liquid fuel."

This is all reflected in how O2 sensors are used in modern engines. ECU's are in open loop mode and just approximating what the fuel mixture should be, 'til things warm up and the fuel burn is consistent and the O2 readings are realiably showing what is happening before switching to closed loop mode where the O2 readings are actually controlling the mixture.