Backfire problems
Thread Starter
Melting Slicks
Joined: Feb 2007
Posts: 2,937
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From: OP Kansas
C2 of Year Finalist (appearance mods) 2019
Backfire problems
I have a situation I can't seem to figure out. My engine is backfiring on deceleration. The harder I let the car brake with the engine rpm's, the louder the backfire. Unfortunately it also does it between shifts.
If I get back on the gas just slightly between shifts, I can avoid it. So to me it seems like it's happening under high vacuum.
I am starting to scare the heck out of other drivers as I come up to a stop light and I arrive with a BANG!. I can't seem to find any vacuum leaks and I don't hear any exhaust leaks either. Anyone have any advise where I should start? Do these plugs look like they are burning too lean? They do seem more white than tan. Here are what my plugs #1 and #3 look like:
This is a new one so there is something to compare the others to regardless of color differences in monitors.
If I get back on the gas just slightly between shifts, I can avoid it. So to me it seems like it's happening under high vacuum.
I am starting to scare the heck out of other drivers as I come up to a stop light and I arrive with a BANG!. I can't seem to find any vacuum leaks and I don't hear any exhaust leaks either. Anyone have any advise where I should start? Do these plugs look like they are burning too lean? They do seem more white than tan. Here are what my plugs #1 and #3 look like:
This is a new one so there is something to compare the others to regardless of color differences in monitors.
Melting Slicks
Joined: May 2001
Posts: 2,749
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From: Dadeville AL
A common cause of popping while using the engine to brake during deceleration is an exhaust leak. Outside air (oxygen) will be drawn into the hot, fuel rich exhaust and cause small explosions. Check for a leak.
Race Director
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From: northern california
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When I was dialing in the Webers in my vintage racer, I experienced decel backfire also. For reasons unrelated to the backfiring, I went to a richer idle jet and, to my surprise, the decel backfiring stopped.
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
Le Mans Master
Joined: Sep 2008
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From: Lake Minnetonka, Mn
2018 C1 of Year Finalist
When I was dialing in the Webers in my vintage racer, I experienced decel backfire also. For reasons unrelated to the backfiring, I went to a richer idle jet and, to my surprise, the decel backfiring stopped.
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
How much richer did you go? What e tubes, main jets and air correctors did you use on those DCOEs?
Race Director
Joined: Feb 2007
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From: northern california
C2 of Year Finalist (track prepared) 2019
Also from memory:
F7 emulsion tubes
170 mains
185 air correctors
65 pump jet
all these work with 38mm chokes.
Jim
Last edited by jim lockwood; Dec 26, 2011 at 09:07 AM. Reason: corrected jetting
Le Mans Master
Joined: Sep 2008
Posts: 5,661
Likes: 2,344
From: Lake Minnetonka, Mn
2018 C1 of Year Finalist
Thanks, Jim. Those are right in line with the 48mm IDAs jettings for 289 Fords. The air correctors are in line, too. The higher the number the more "braking" at higher RPM.
Thanks.
Thanks.
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Race Director
Joined: Jan 2003
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From: Central Arkansas
When I was dialing in the Webers in my vintage racer, I experienced decel backfire also. For reasons unrelated to the backfiring, I went to a richer idle jet and, to my surprise, the decel backfiring stopped.
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
Sooooo, if you have control over the idle/low speed mixture, try enrichening it and see if that helps.
Jim
Pro
Joined: Sep 2010
Posts: 509
Likes: 141
From: Southeast, pa.
back-firing
i had the same problem just last month. i narrowed it down to the exhaust valve on my #1 cylinder(carbon build-up). while the car was in for repairs i had the mechanic do all the valves---while they had it apart it would save tearing it down again & again & again.
Melting Slicks
Joined: Oct 2006
Posts: 2,366
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From: Flower Mound TX
Don't discount a burnt plug wire. I had that happen just last week for the second time this year. It would backfire during a shift and a little harder gas pedal. The funny thing is that it started on my way down to a Veteran's Day Parade recently. As I went under a viaduct I shifted and heard a loud "Bang". I thought I was going to scare everyone.
In my case, I have headers. I am now more aware of this. When you are driving down the road and step on the gas, does the shifter shake a little?
In my case, I have headers. I am now more aware of this. When you are driving down the road and step on the gas, does the shifter shake a little?
Race Director
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Jim
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How many miles are on the motor? If it's a high miler, the timing chain could have enough stretch in it, that your timing could change on de-aceleration. Just an idea......
Burning Brakes
Joined: Nov 2001
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From: Tank sticker? I ain't got no tank sticker! I don't need no tank sticker! I don't have to show any stinking tank sticker!
You are getting unburnt gasoline in the exhaust system where the hot exhaust from other cylinders ignites it in the exhaust pipe/muffler.
Could be a bad exhaust valve or bad cylinder ignition source (plug, wire, cap tracking).
Could be a bad exhaust valve or bad cylinder ignition source (plug, wire, cap tracking).
Thread Starter
Melting Slicks
Joined: Feb 2007
Posts: 2,937
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From: OP Kansas
C2 of Year Finalist (appearance mods) 2019
I'm embarrassed to say I found my problem and many of you got it right. I had an exhaust leak at the manifold to side pipe connection. Funny thing is, I couldn't hear it or see it, but so many people said it sounds like and exhaust leak I had to try and find one.
What I did was let the car cool down and then start it up and get under the car and feel around with my hands before it got too hot to do it and I found the leak on the engine side of the connection at the flange. Once I fixed it the car stopped the backfire and also seemed to run better.
Thank you all very much for your help.
BTW, here is the result of the engine swap.
Before:
After:
Here is a set of pics I took during the process if anyone is interested.
Engine Swap
.
What I did was let the car cool down and then start it up and get under the car and feel around with my hands before it got too hot to do it and I found the leak on the engine side of the connection at the flange. Once I fixed it the car stopped the backfire and also seemed to run better.
Thank you all very much for your help.
BTW, here is the result of the engine swap.
Before:
After:
Here is a set of pics I took during the process if anyone is interested.
Engine Swap
.
Last edited by KC John; Dec 26, 2011 at 04:33 PM.
