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Cameo Ivory 1967
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Discussion Starter · #1 ·
After a 2 year battle with my exhaust, I finally have it sounding right, but now I have afterfire (usually misdescribed as backfire) when I shift.

I always believed this to represent a lean condition, but that's not what the internet is saying!

So, my car didnt have it before, and now that I opened up the exhaust, I do have it. Any ideas for tuning it out? Anyone else with the issue?
 

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1967 Lemans 1967 GTO parts car
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I have been driving with the teen in the Gray ubiquitous blob getting passed by many a car on the two lane country roads. I was surprised how many cars do that when they let off after passing. The teen calls them car farts. Cars you cant hear the exhaust but you hear the popping. Its been an embarrassing two weeks and she is legal now. Time to buy her a 4x4 truck.
 

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1968 GTO 400, TH400
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I think I may have fought with a similar issue on an old Bronco, I would get crackle pops bordering on backfire volumes out the exhaust on decel or off throttle conditions, I changed from Flowmaster mufflers to Dynomax turbo mufflers and that helped some (a little more restrictive exhaust flow) but the most improvement came when I changed fuel injection to the Edlebrock set up that allowed me to richen up the fuel mixture on decel conditions, so long story short carbureted richen the idle circuit, fuel injected the same or add fuel to decel ?
 

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1967 GTO
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Does it happen more when you're really getting on it? Or does it happen when you're cruising as well?

If it's happening when you're shifting hard through the gears, my understanding of it is you create a momentary rich fuel mixture when you left off the gas abruptly--which burns incompletely then makes it's way to your exhaust where it ignites. It's exacerbated by shorter or higher flowing exhaust.

It's not apples to apples, but I get the same thing on my R1 with my Coffman shorty exhaust. A tune helped it a bit, but there's no getting around it completely in my experience.

But I think that's just one possible reason. This video helped me wrap my head around the other factors:
 

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Cameo Ivory 1967
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Discussion Starter · #7 ·
Does it happen more when you're really getting on it? Or does it happen when you're cruising as well?

If it's happening when you're shifting hard through the gears, my understanding of it is you create a momentary rich fuel mixture when you left off the gas abruptly--which burns incompletely then makes it's way to your exhaust where it ignites. It's exacerbated by shorter or higher flowing exhaust.

It's not apples to apples, but I get the same thing on my R1 with my Coffman shorty exhaust. A tune helped it a bit, but there's no getting around it completely in my experience.

But I think that's just one possible reason. This video helped me wrap my head around the other factors:
All of my R1's and FZR's do it... Cycles are much more prone.

It only does is when cruising, and only when pushing in the clutch. If I shift fast it does not do it.
 

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Cameo Ivory 1967
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Discussion Starter · #8 ·
I have no idea, but I'm curious because I often hear that with the local bikers as they let off throttle to shift.
For sure motorcycles always do it... My theory was that it was because when you let off the throttle, in gear, the engine kept turning, therefore continuing to draw in air, but no fuel... however, that was just a guess.

Now my GTO has a very aggressive exhaust, and thats what started it.
 

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Cameo Ivory 1967
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Discussion Starter · #10 ·
Actually, with motorcycles; you can turn off the ignition but leave the key on while driving... and it's a load of fun to zip down the road, flip the ignition off, but stay on the throttle. Then when you flip it back on, it sounds like a mortar round. It's a sure fire way to cause a pedestrian to drop a deuce.

So I guess that it it unburnt fuel igniting, but why would a free flowing exhaust bring it on... unless it's the improved scavenging?
 

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For sure motorcycles always do it... My theory was that it was because when you let off the throttle, in gear, the engine kept turning, therefore continuing to draw in air, but no fuel... however, that was just a guess.

Now my GTO has a very aggressive exhaust, and thats what started it.
What exactly did you do to make your exhaust sound more aggressive?
 

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Discussion Starter · #14 ·
After two full exhausts, two different x pipes, open pipes, and three sets of mufflers in a year... I finally bought the loudest exhaust made for the car... and since I was skeptical that it was going to be loud enough, I got a kit with cutouts built into it.

Sure enough, it wasnt nearly aggressive enough... and unfortunately, with the cutouts open, it was too LOUD and sounded like crap!!!!

So... I made my own baffles/ mufflers out of straight pipe, and bolted them to the cut outs. NOW ITS PERFECT!!!!!!!!!! No more being embarrassed by the Chevelles and Mustangs.
Bicycle part Automotive exterior Auto part Font Fashion accessory
 

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I think you are a bit lean lean Army. Yes rich can do it also. But lean is more common. At cruise with a light pedal you are getting a lot of timing which you need to burn the lean cruise mixture. But yours may be a tad too lean for your setup.

when you slowly push the clutch down you also slowly “Decrease the load” on the engine by letting your foot off the gas pedal. Youkinda work those feet at the same speed. When you clutch easy your foot comes up easy andwhen you shift hard your foot comes up fast.

So depending on the speed where this happens an enrichment of that carb circuit may be called for.

say at 3000 Rpm you are at 60 mph light throttle. That will be on your main jets and idle circuit. ( because idle circuit never really stops, it is just overdone by th email jets.

then I would increase the main jets two sizes and see if the after fire changes.…then see what the AFR is if it is gone.
 

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1968 GTO 400, TH400
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If you have an AFR gauge in there keep an eye on it while shifting and see if its changing momentarily from the cruise readings and in which direction

Sounds a lot like my Bronco, it did it worse on solid to aggressive acceleration and with the truck tranny, shifts were never real fast and it would let out some pretty solid backfires (for lack of a better word) if I didn't feather the gas between shifts
 
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