Backfiring when letting off the gas

[ghjedi]

I have a slight backfire condition that is really starting to get to me.  My car runs great at idle and even during normal acceleration without any hint of backfire what so ever.  But, if I really jump on it, I can experience a one-time backfire while transitioning from 1st to 2nd gear.  I have a 4 speed manual transmission and when I let off the gas for split second around 3500 rpms or so, I can hear a fairly loud bang with a slight loss of power. Once I get into 2nd gear and continuing accelerating, or even as I go from 2nd to 3rd, it does not happen again.  If I drive with normal acceleration, the one-time backfire doesn't happen.

I tried adjusting the carb and timing without much luck-- I just can't seem to eliminate it.  I have a 440 engine with a Proform 850 carb.  Someone suggested that adding a crossover pipe on my dual exhaust system will solve my problem (I don't any cross over pipes), but from what I've read, I'm not sure.

Any thoughts on what might be causing this or what else I should check?  It's kind of embarassing when you're trying to show off what a 440 is all about   

[mopar12372]

check for exhaust leaks around the engine and at the lower part of the exhaust sysem . a slight leak could cause this . second what fuel pump are you using ? what fuel pressure is the system set at ? does it happen only when you let off the throttle or does it seem to happen when under a load when at wide open throttle hotdogging it?
 did you set the air fuel mixture with a vaccuum gauge ?  is the float level set right , if its too low you can run into a lean condition .
ok now , what is  you engine combo? cam , heads , intake , carb , ignition, fuel pump , exhaust , compression.
lotsof mixed questions , just trying to make you think . good luck -->Bill

[PlumCrazyRTSE]

When I first got one of my Challengers a couple years ago, it had a backfire that would occur sometimes when I let off the gas.  First thing I checked was the plug wires and firing order and I found that the previous owner had two wires swapped and another wire was barely attached to the connector at the plug.  With a slight tug, the wire pulled right out of the connector boot.  After I fixed those two issues, it immediately stopped backfiring and also ran a heck of a lot smoother.  Can't imagine why.   

[ghjedi]

check for exhaust leaks around the engine and at the lower part of the exhaust sysem . a slight leak could cause this . second what fuel pump are you using ? what fuel pressure is the system set at ? does it happen only when you let off the throttle or does it seem to happen when under a load when at wide open throttle hotdogging it?
 did you set the air fuel mixture with a vaccuum gauge ?  is the float level set right , if its too low you can run into a lean condition .
ok now , what is  you engine combo? cam , heads , intake , carb , ignition, fuel pump , exhaust , compression.
lotsof mixed questions , just trying to make you think . good luck -->Bill

Good questions and yes, it only happens when I let off the gas, particularly when shifting gears.  If I am continuously accelerating slow or fast, it does not happen, nor does it happen at idle.  It also does not happen if I have the car in neutral and rev it high either slowly or quickly.  If I take it easy on the acceleration and then shift from 1st to 2nd, it will usually not happen.  It does not happen all the time, but when it does it only seems to happen while driving and it is a pop sound similar to a muffled shotgun blast.  It is never more than 1 pop.

To answer your other questions, I have a 440 engine with the stock fuel pump.  It also has electronic ignition and it appears to be the stock setup for this year Challenger.  I don't think anything special has been done to the 440 in terms of cam or compression and I can't be sure, since I'm not the original owner.  I recently put a Mopar intake manifold on the engine and swapped out a prior Edelbrock 650 carb for the Proform 850 street series carb. I also had to have the exhaust pipes replaced due to leaks and worn mufflers.  I have new flowmasters and the engine has headers from before.  Note that I did not have backfiring problems prior to the intake and carb changes.  There doesn't appear to be any vaccum leaks at the carb and the engine runs quite smooth and strong, except for this ocasional problem. With the Proform carb, I can see see through the little window and the fuel levels look okay (not too high nor too low)

One question for you, have you ever heard of the lack of a crossover pipe causing a backfire?  From what I've read, it only appears to solve for a small power increase and possibly a reduction in small popping noises in the exhaust from back pressure.

[ghjedi]

When I first got one of my Challengers a couple years ago, it had a backfire that would occur sometimes when I let off the gas.  First thing I checked was the plug wires and firing order and I found that the previous owner had two wires swapped and another wire was barely attached to the connector at the plug.  With a slight tug, the wire pulled right out of the connector boot.  After I fixed those two issues, it immediately stopped backfiring and also ran a heck of a lot smoother.  Can't imagine why.   

Wow, now that is interesting and something for me to think about.  I did replace the plugs and wires awhile back and I was careful to replace 1 at a time so as to preserve the firing order that the car had before.  I did not verify that this firing order was correct per the manual. 

I have messed up firing orders before on other cars, but that was usually pretty easy to identify, since the car ran poorly all the way around. 

PlumCrazyRTSE, are you saying that your Challenger ran great except for the ocassional backfire with the mixed up firing order? 

[usraptr]

Since it did not do it before the carb change, my guess is that you are dumping raw fuel into your hot exhaust system.  With the headers, no X or H pipe and flowmasters, you have little if any back pressure.  This lack of back pressure and extra fuel will cause the back firing.  This is very common on Harleys.  Owners take off the stock exhaust system with the built in X/H pipe and put on straight pipes to make them louder.  Then they will back fire when letting up on the throttle and they actually loose horsepower.  You need some back pressure in the system to help with the scavenging of the exhaust gases from the heads.  So without re-jetting your carb, I think you best bet is an X/H pipe and even possible a muffler change to something a little more restrictive than flowmasters, like Magnaflows.  And you'll also pick  up some horsepower along with getting rid of the back firing!

[mopar12372]

if you have duel exhaust without a crossover pipe i witll do this assuming everything else is in good order. did you go up in size with the exhaust too?
i would adjust the air fuel ratio , you may have to jet the carb to your current set up. a up grade isnt a bolt on and go all the time , sometimes you have to make adjustments. when you replaced the intake manifold and replaced the exhaust it allowed the engine to breath better

[ghjedi]

Thanks guys for the suggestions and what you say makes a lot of sense.  I did replace the exhuast pipes and mufflers before the carb and intake change and I do believe that a slightly bigger pipe was put in.  I did not have backfire problems then.  I think you're right about not enough back pressure now, with the bigger pipes, bigger carb, and better intake manifold.

Any thoughts on X vs. H, given my liklihood of low back pressure?

Also, I did check the firing order and it is way off from what the manual says for a 440 engine.  I tried putting it right per the manual and I could not get it to start except for one loud bang.  I put it back the way it was and the car starts up again.  I think I need a drink.

[usraptr]

Not sure if it's possible on Mopars, but do you have the Dist' 180 degrees off?

Not real sure of the difference bet an X and an H pipe.  Might want to google it.

[ghjedi]

Not sure if it's possible on Mopars, but do you have the Dist' 180 degrees off?

Not real sure of the difference bet an X and an H pipe.  Might want to google it.

I don't know, so I'll have to check that.  I wrote down the firing order that was working for me before I played with it, so I'll share here and see if it helps with the diagnosis.  I was comparing it to what my '72 Dodge shop manual says for a 440 engine.  My Chilton's book also confirmed the same for a Dodge 440 engine.

[MEK-Dangerfield]

If the distributor was 180* out, your car would never start.

[shadango]

Also, I did check the firing order and it is way off from what the manual says for a 440 engine.  I tried putting it right per the manual and I could not get it to start except for one loud bang.  I put it back the way it was and the car starts up again.  I think I need a drink.

Is the actual firing order off or are you saying that the locations on the dist cap are way "off"?

[ghjedi]

It is very odd indeed.  If it was just off by position, I would not be too worried about that.  But, it appeared to be off on both counts-- the position on the cap for #1 and the subsequent order moving in a counterclockwise direction from the #1 position.  I'm going by memory now, since I don't have the pattern handy right now.  I'll update this post later tonight with that info.

[ghjedi]

Since it did not do it before the carb change, my guess is that you are dumping raw fuel into your hot exhaust system.  With the headers, no X or H pipe and flowmasters, you have little if any back pressure.  This lack of back pressure and extra fuel will cause the back firing.  This is very common on Harleys.  Owners take off the stock exhaust system with the built in X/H pipe and put on straight pipes to make them louder.  Then they will back fire when letting up on the throttle and they actually loose horsepower.  You need some back pressure in the system to help with the scavenging of the exhaust gases from the heads.  So without re-jetting your carb, I think you best bet is an X/H pipe and even possible a muffler change to something a little more restrictive than flowmasters, like Magnaflows.  And you'll also pick  up some horsepower along with getting rid of the back firing!

Hi, just a clarification about what you said here-- do you think my problem is too much back pressure or not enough of it?  I'm asking because from what I've read, going up in exhuast pipe size might allow the hot exhuast gasses to cool faster than optimal and thus slow down the flow out (i.e some back pressure generated).  I also thought the X or H pipe help to equalize the pressure across pipes and reduce back pressure.

Assuming that's the case, then is my problem more about too much back pressure during deceleration (lower RPMs)?  I've never had to delve into exhuast setup at this level of detail for my other stock cars, so I'm trying to understand this better.

Thanks

[shadango]

It is very odd indeed.  If it was just off by position, I would not be too worried about that.  But, it appeared to be off on both counts-- the position on the cap for #1 and the subsequent order moving in a counterclockwise direction from the #1 position.  I'm going by memory now, since I don't have the pattern handy right now.  I'll update this post later tonight with that info.

I dont want to muddy the waters but are you sure that your are going in the right "direction" when you are looking at the current firing order? Dont big block disturbutors rotate in the opposite direction (CCW) compared to a small block (clockwise)?  Maybe you are just looking at it "backwards" ?