Side pipe back fire
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Joined: Aug 2018
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Side pipe back fire
Just installed a set of side pipes on my 78 l82 and first off I love the way they look and sound except for one sound. I'm getting a slight back fire on deceleration it's a not very loud but a crisp pop. Also at about 2500 rpm the engine surges and pops I've tuned the idle mixture needles and no change. The baffles I'm running is the hooker reverse flow, I know they kill a lot of power but I don't want my car to be insanely loud. I was thinking those might be causing me to run rich but I wanted to see what you guys thought.
Team Owner
Joined: May 2001
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From: Western NY
Just installed a set of side pipes on my 78 l82 and first off I love the way they look and sound except for one sound. I'm getting a slight back fire on deceleration it's a not very loud but a crisp pop. Also at about 2500 rpm the engine surges and pops I've tuned the idle mixture needles and no change. The baffles I'm running is the hooker reverse flow, I know they kill a lot of power but I don't want my car to be insanely loud. I was thinking those might be causing me to run rich but I wanted to see what you guys thought.
Le Mans Master
Joined: Mar 2004
Posts: 7,423
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From: Little Rock AR
Agreed - Recheck the header bolts. They loosten themselves especially when new and need to be retightened a few times before they stay tight.
BTW - The term "Backfire" refers to popping up out the carb. "afterfire" is popping out the exhaust.
If the gaskets are not leaking at the head, you may be experiencing a lean misfire causing the afterfire. Thats when the throttle is closed quickly and you get lots of air bypassing the edges of the throttle blades without enough gasoline to ignite the mixture properly. The mixture will misfire, and the unburned fuel ignites in the hot exhaust stream causing the tiny popping explosions you hear.
If it did not afterfire before the headers were installed (without any other carb changes), my money would be on an exhaust leak at the header to cylinder head joint.
BTW - The term "Backfire" refers to popping up out the carb. "afterfire" is popping out the exhaust.
If the gaskets are not leaking at the head, you may be experiencing a lean misfire causing the afterfire. Thats when the throttle is closed quickly and you get lots of air bypassing the edges of the throttle blades without enough gasoline to ignite the mixture properly. The mixture will misfire, and the unburned fuel ignites in the hot exhaust stream causing the tiny popping explosions you hear.
If it did not afterfire before the headers were installed (without any other carb changes), my money would be on an exhaust leak at the header to cylinder head joint.
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Pro
Joined: Mar 2014
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From: Texas
Round port heads with Rectangular port header......nothing can go wrong there......
That would give most men an excuse to buy a new cam and set of aluminum heads.........
That would give most men an excuse to buy a new cam and set of aluminum heads.........
Last edited by Gunfighter13; Aug 31, 2019 at 10:26 AM.
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I've checked the header bolts they aren't loose yet and I did previously have the oe exhaust system and it was running good and my engine is all stock. I pulled the plugs today and they are all white so I'm thinking you guys are right it's Lean so I guess to main jet sizes?
Instructor
Joined: Jul 2016
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From: Poway CA
Before you start changing jet sizes, which you may end up having to do, pull off your air pump belt (if your headers have the A.I.R. tubes hooked up to them) and see if not pumping in air into your headers makes any difference. Your diverter valve or the vacuum line it uses may be causing an issue. It's an easy and quick check.
Team Owner
Joined: May 2001
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From: Western NY
I've checked the header bolts they aren't loose yet and I did previously have the oe exhaust system and it was running good and my engine is all stock. I pulled the plugs today and they are all white so I'm thinking you guys are right it's Lean so I guess to main jet sizes?
Team Owner
Joined: May 2001
Posts: 87,294
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From: Western NY
Holley makes a "spread bore replacement" carb, that has the same throttle bore spacing as the Q-Jet, so you can retain the OE intake manifold. I don't know the model number offhand, but the "mail order speed shops" have it listed in their catalogs.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Afterfire in the exhaust on decel is a lean condition. Increase jet size 2 sizes (unless you already have 77 jets in it), raise the APT setting as outlined in my carb setup paper, and get your float level set to .300". Make sure idle mixture screws are not set too lean. Going from a restrictive stock exhaust to a free-flowing true dual setup will lean you out, and it will make it pop in the exhaust. Richen it up.
Lars
Lars
Last edited by lars; Sep 1, 2019 at 03:56 PM.
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So I just looked at your paper and it says for my number carb it says it comes stock with a 77 so if it already has 77 jets in it when I take it apart would I have to use metering rods to richen it? It's that correct?
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
Before you start changing jet sizes, which you may end up having to do, pull off your air pump belt (if your headers have the A.I.R. tubes hooked up to them) and see if not pumping in air into your headers makes any difference. Your diverter valve or the vacuum line it uses may be causing an issue. It's an easy and quick check.
His headers don't have fittings for that garbage, thankfully.
Last edited by 7t9l82; Sep 1, 2019 at 05:40 PM.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
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Last edited by lars; Sep 2, 2019 at 10:14 AM.
