# Cause of exhast backfire?



## leeklm (Mar 11, 2012)

When letting up on the gas and using the engine to brake (manual trans), I will typically get somewhat lound "popping" or backfire in the exhaust. I had always assumed this was caused by the car running rich, and unburned fuel igniting in the exhaust.

Does this sound like the case, or could there be other causes?

engine is a low compression 400 with headers and turbo style mufflers.


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## lars (Sep 28, 2004)

Actually, it's the opposite: Popping in the exhaust on deceleration with the throttle closed is caused by a lean condition which induces occasional lean misfire. The lean misfire will put unburned air/fuel into the exhaust, where it then explodes. Richen up your carb and it will go away (particularly idle and transition circuits). If you have headers, the carb will run lean unless you have richened it up to compensate for the free-flowing exhaust.

Lars


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## geeteeohguy (Feb 2, 2008)

Exhaust leaks at the headers can also contribute to this, by adding enough O2 to re-ignite unburned gasses in the exhaust system


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## leeklm (Mar 11, 2012)

That may help explain the general lack of part-throttle response with primaries, and a big power gain with a shot from the pump, and transition to the secondaries. I am currently building/experimenting with two different qjets, using Cliff's book & parts, Lars paper, and some trial & error. 

The current "lean" carb is using #70 jets and .049 rods (or .50, cant recall). The new carb under assembly is using #72 jets and .047 rods, but also has pullover enrichment, so probably not a fair comparrison, although this carb was more responsive when testing before the rebuild process.

I need to refer back to your paper for the jet/rods fuel delivery calculations, and review my various circuit settings from cliffs book... 

Thanks for the input.


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## leeklm (Mar 11, 2012)

geeteeohguy said:


> Exhaust leaks at the headers can also contribute to this, by adding enough O2 to re-ignite unburned gasses in the exhaust system


and i do have plenty of those... pos headers.


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## geeteeohguy (Feb 2, 2008)

In that case, I would fix the exhaust leaks first and test drive. That may take care of the problem with the backfire.


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## lars (Sep 28, 2004)

...although that 70/49 jet/rod combo is super-lean. I think he's going to get exhaust popping no matter how tight those headers are with a lean-out condition like that. If the WOT mixture/performance is OK with the 70 jets, drop those rod sizes down to about a 41-or-so and see how it responds. If it were me (and not knowing the carb number you're working with), I'd bump jetting up 3 sizes and drop the rod size 4 sizes to a 73/45 combo as a starting point.

How high is the float level?

Lars


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## leeklm (Mar 11, 2012)

Float was about .3", or just a touch lower than the factory spec of .25, and I was running out of fuel at 4K in 2nd gear. Moved the float a little higher than factory spec and seems to have taken care of the high rpm fuel issue, but now getting some gas vapor smell after shut down (but no signs of fuel leaking), so thinking I need to move it down a little again. This carb is from a 73 Olds Toronado 455.

The carb-in-process is a 7042264 (72 Pont 400) with the 74/47 primary, and should be assembled in the next day or so for a trial. I definitely need to order extra jets & rods, as I only have the two sets for the qjet! 

On this rebuild, I am unable to get the ball seated tight in the fuel well. When testing with carb spray, the fluid does not "run" into the pump cavity, but does seep into the cavity over a minute or so. Is this anything to be concerned about?


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## leeklm (Mar 11, 2012)

Bolted the fresh #264 carb on and runs excellent! Idles perfect, no hesitation, maybe a very slight bog with secondary (thinking maybe need to tweak flap spring) Backfire is much better, only pops a little now if I leave it in first gear and braking from a higher rpm. By the way, the Olds carb had 53 rods with the #40 jets!! 

However, I recently learned from my new PHS document that my car originally had a tri power, so I may now have to learn my away around the 2bbl... The Wife is real excited about that 

Thanks to Lars & Cliff with the carb help, the car is running great!!


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## lars (Sep 28, 2004)

The 7042264 carb is a '72 400 California carb, so with the 74/47 stock jetting, it's still lean for your 400 equipped with headers and free-flowing exhaust. Bump that primary jetting up 2 sizes. 

Also, assuming you have the stock "CR" rods on the secondary side: Those "CR" rods are pretty fat, with a .055" metering tip. The 67-69 GTO performance carbs ran a "BE" or "BF" rod, which have tips in the .040" - range. If you feel like you're getting a secondary tip-in stumble, try fattening up that secondary side with an Edelbrock "CE" rod (EDL part number 1951). This is a .0413 long-tipped rod that will give you a proper enrichment on the secondary side.

Lars


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## NorwalkNugget (Aug 5, 2011)

Darnit this is happening to me since I put hedman headers on the car. It only happens on decelleration off a steep decline exit ramp when its kinda cool out with a hint of moisture/haze in the air. Only time I get a HUGE BANG right underneath drivers seat. I would say it's happened 3 times since installing the headers and flowmasters.

I have a holley 4555 carb.


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## lars (Sep 28, 2004)

As noted earlier, headers will lean the engine out. If the carb was set up correctly before the header installation, it will be too lean with the headers, and it will cause a lean misfire and exahust popping on deceleration. Headers require an increase in carb jetting by at least 2 jet sizes - often more.

Lars


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## leeklm (Mar 11, 2012)

lars said:


> The 7042264 carb is a '72 400 California carb, so with the 74/47 stock jetting, it's still lean for your 400 equipped with headers and free-flowing exhaust. Bump that primary jetting up 2 sizes.
> 
> Also, assuming you have the stock "CR" rods on the secondary side: Those "CR" rods are pretty fat, with a .055" metering tip. The 67-69 GTO performance carbs ran a "BE" or "BF" rod, which have tips in the .040" - range. If you feel like you're getting a secondary tip-in stumble, try fattening up that secondary side with an Edelbrock "CE" rod (EDL part number 1951). This is a .0413 long-tipped rod that will give you a proper enrichment on the secondary side.
> 
> Lars


Secondaries are CS, which I assume someone replaced along the line... I measured the CS, and looks to be in the .040 range. My CJ rods are also around .040. When both rod series are at .040, Is the difference in the rate at which the rods allow fuel to pass? I was looking to the chart in Cliff's book regard the various degree of the rods, but I have not totally grasped the concept yet of what it all means. I assume it is the measurement of fuel applied based on secondary position?


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## SDGoat619 (Mar 21, 2011)

I am having a similar issue however my backfire seems to only come under heavy throttle. Its an 800cfm eddy carb this tends to only happen with the choke on does this mean that i am to rich??

The throttle responce is way better with the choke partially on i guess my question is that should i move up one size richer so that i can pull the choke off asap once warm. I live in San Diego so I really shouldnt need to fuss with a choke to often.... sorry to hijack the thread just thought it would be a pertinent place to ask this question.

Thanks

Nick


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