Popping/backfire through exhaust on loaded downshift - Team Camaro Tech
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  #1  
Old Aug 6th, 10, 04:31 PM
melsman melsman is offline
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Default Popping/backfire through exhaust on loaded downshift

Hello All,

I've done a lot of searching on this forum and others on this topic, and I know there have been a lot of answers, but I'm not sure I have the same issue. Or at least, others that I've read have not described the same symptoms.

I have a '68 Camaro with a pre-81 GM Goodwrench crate engine (350ci - made in Mexico type) with Hooker headers, Flowmaster exhaust (2.5"), an Edelbrock 1806 Carb, Mallory HEI (nothing fancy), mild cam, and Accel wires. Timing is set at 6* BTDC and have a strong spark. No discernible exhaust leaks. Muncie M-21 tranny.

I have strong, smooth acceleration under load, and smooth no-load deceleration (clutch in, load off engine - decelerates quietly and smoothly). However, if I downshift and decelerate under load (engine braking), I get all kinds of popping and backfiring through the exhaust.

This is a recent occurrence, and not associated with any engine work. I'm guessing that the solution is probably a simple one that's right under my nose, but I'm kinda at my wits end.

Any ideas?

Melsman
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  #2  
Old Aug 6th, 10, 05:37 PM
Nashville Beth's Avatar
Nashville Beth Nashville Beth is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

I don't think you have enough initial timing - 12 degrees is a good starting point. Move your distributor counterclockwise to advance the timing.

Do you have the vacuum advance hooked up ? It should be hooked up to manifold vacuum.

See if you can do this without the engine pinging and go from there.
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  #3  
Old Aug 6th, 10, 06:36 PM
melsman melsman is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Thanks for the input, Beth.

Yes, I have the vacuum connected - until I time it, at which point I plug the vacuum line and set the timing at idle.

Speaking of that... are you talking about 12* ATDC? You said to "advance" the timing from the current 6* BTDC. That doesn't seem correct to me, but I'm no expert.

Melsman
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  #4  
Old Aug 6th, 10, 06:57 PM
Z15CAM Z15CAM is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Agree you need more Initial Advance also if you have pin holes before the mufflers the unburnt fuel in the pipes, with the throttle closed, will super heat from the inrush of air and ignite - a good way to blow the mufflers too ;o)

Sometimes using a 1 quart Vacuum Reservoir "T'd" into the Vacuum Advance hose can help eliminate this condition.

Stock Initial is approx 8 degrees BTDC. Generally a SBC will take 12 to 14 degrees BTDC Initial with some modifications - Every engine is different. When setting Initial Advance (which is where an engine will start and idle by turning the distributer) ensure the Carb is setup with throttle plates in the idle circuit, the Secondary Plates set per Mfgr's Specs and Idle Air Screws approx 1 1/8th turn off seat. Disconnect all Vacuum sources such as Vacuum Advance, Tranny Modulator Sx, Power Brakes and PCV - The last thing you want is a Vacuum leak when establishing Initial Advance.

Once you have establish the initial advance write it down for future reference and take a Vacuum Reading. The Power Valve and Distributer Vacuum Pod should have approx 1.5 to 2 Hg's below the engine's Vacuum at Idle. If you note the Timing with Vacuum Advance haunting on the Damper it probably means you have Too High of a Vacuum Pod for the Engines Vacuum at Idle or you have a Vacuum leak - This is why you plug all Vacuum Sources to ensure there are no Vacuum leaks when establishing Initial Timing for a particular Engine.

Now connect the Vacuum Advance and slightly adjust the Throttle Screw and Idle Air Screws to compensate for the Vacuum Advance. The Timing at idle should be approx 10 or 12 more (say 24 degrees) at idle with full vacuum advance and the idle air screws set approx 3/4 turn off seat.

Drag Racers are not that concerned about Initial Timing and concentrate on the Total and getting it all in at a specific RPM but for the Street, Initial Timing is a MUST.

I also recommend that you use Manifold Vacuum to the Distributor Advance Pod and Plug the Carburetors Port Vacuum source. Port Vacuum is for SMOG Engines.
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Last edited by Z15CAM; Aug 6th, 10 at 07:49 PM.
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  #5  
Old Aug 6th, 10, 07:51 PM
melsman melsman is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Hi Z15CAM,

Thanks for all of the info. I need to do more reading and research to completely understand everything you wrote. Thanks for taking the time!

And Beth... it finally dawned on me what you were trying to tell me... Sometimes my brain ain't so quick...

Melsman
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  #6  
Old Aug 6th, 10, 08:36 PM
Steptoe Steptoe is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Intial will not make any difference to the backfire...its what is in the curve at the rpms you are doing and what is in the VA...as the engine is on full vaccuum.
Intial means squat other than how well it idles and NOx emissions
upping thwe intial could over advance the normal running causing detonation etc
What is the total?
intial +cent and intial +cent +VA

Backfire is normally a lean mixture or sevier scavenging.
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  #7  
Old Aug 6th, 10, 08:48 PM
Straight-line-69 Straight-line-69 is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Vacuum leak
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  #8  
Old Aug 6th, 10, 09:00 PM
Steiner Steiner is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Sounds like idle mix is a bit lean. Once the throttle closes, that is what is supplying the fuel. A bit too lean of a mix and you get incomplete burn so the unburned fuel gets into the hot pipes and pop pop pop.

Used to happen all the time on a carbed bike whenever changing out to a new set of pipes even after installing new jets. Another half turn out on the idle mix screws used to cure it. I was always too lazy to use the four dial synchronizer.
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  #9  
Old Aug 6th, 10, 11:30 PM
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Default Re: Popping/backfire through exhaust on loaded downshift

Quote:
Originally Posted by steptoe View Post
intial will not make any difference to the backfire...its what is in the curve at the rpms you are doing and what is in the va...as the engine is on full vaccuum.
Intial means squat other than how well it idles and nox emissions
upping thwe intial could over advance the normal running causing detonation etc
what is the total?
Intial +cent and intial +cent +va

backfire is normally a lean mixture or sevier scavenging.
Quote:
Originally Posted by straight-line-69 View Post
vacuum leak
bingo!!!!!

Vacuum leak!
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  #10  
Old Aug 7th, 10, 03:18 PM
melsman melsman is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Thanks for all of the responses, folks! I appreciate it.

I played with the Initial Advance, and found that the engine liked running about 18* BTDC(!) - good idle, good start, and no pinging. And no more popping on loaded deceleration.

Melsman
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  #11  
Old Aug 7th, 10, 03:37 PM
Steptoe Steptoe is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

lean mixtures (which a vac leak causes) requires more advance.
So the question is have up fixed or patched the problem.
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  #12  
Old Aug 7th, 10, 09:06 PM
wagonman's Avatar
wagonman wagonman is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

I'm lost as well.
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  #13  
Old Aug 7th, 10, 11:06 PM
ace's68 ace's68 is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

I don't see how it is physically possible to get a lean pop out of the exhaust from the lack of fuel? that would mean what? The cylinder is igniting while the exhaust valve is open? You would def. hear that through the motor as detonation. I am confident that it is un burned fuel going through a hot exhaust, and the exhaust is probably a little too big and open for the power you are at. If it's a lean condition, why don't stock trucks with redneck pipes do it on de cel? Assuming the truck is computer controlled here, as an example.
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  #14  
Old Aug 8th, 10, 02:18 PM
Steptoe Steptoe is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Quote:
I don't see how it is physically possible to get a lean pop out of the exhaust from the lack of fuel?
That what everyone says when they 1st come across it...detailed explantion in older post.
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  #15  
Old Aug 8th, 10, 11:18 PM
ace's68 ace's68 is offline
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Default Re: Popping/backfire through exhaust on loaded downshift

Quote:
Originally Posted by Steiner View Post
Sounds like idle mix is a bit lean. Once the throttle closes, that is what is supplying the fuel. A bit too lean of a mix and you get incomplete burn so the unburned fuel gets into the hot pipes and pop pop pop.
So when the throttle closes, the last bit of the secondary circuit is getting an incomplete burn from going back to the idle circuit...? I'm still confused a tad.
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