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Backfire in the exhaust when decelerating


Sly_drums
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Since I have my Mustang I always get backfire in the exhaust when decelerating. I know it can be a timing problem but what else ? It did it when I had the 2 barrel carburetor (and at that time the car seemed properly tuned), now I have a Holley 4 barrel and it's the same. What do you suggest to check ?

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If it's the same and no worse with the 4 barrel than the 2 barrel could be an intake valve hung open by carbon build up causing exhaust stroke detonation. Get some Sea Clean and pour into the carb as recommended. Make sure the car is NOT in the garage. May have to repeat it after you drive it a while depending on the build up. Cheapest try.

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Just cruising along minding our own business when BAM!!! The LAWS show up.

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It's banging and it's a bit worse with the 4 barrel I have to say.

My exhaust is really poor, I need to change it and I'm sure there are some tiny leaks that can cause this but not sure. I will try what you said, thanks.

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Depending on the type of exhaust system your running. (let us know) could be a back pressure issue. I recall you've done a lot of work. But why don't yo post up again what you heve and that will help in solving the problem without guessing.

2rr7aiv.png

 

Just cruising along minding our own business when BAM!!! The LAWS show up.

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You're right, here we are :

 

72 Mustang Grande

351 Cleveland 2V with WEIAND X-Celerator 4V intake

 

Holley 600 CFM carb since a few days (we tried to tune it properly and seems a lot better now)

Exhaust is a "custom" one, done by the previous owner, but it's a mess really, this will be the next thing I'll change for sure. I think there are a few leaks here and there. I'd like to get new exhaust headers + Flowmasters etc. Still have the original headers at the moment.

 

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If the 4 barrel made it worse, I'de still be looking at carbon build up on a valve since your supplying more fuel. New exhaust is still a great idea. Let that rascal breath!!

2rr7aiv.png

 

Just cruising along minding our own business when BAM!!! The LAWS show up.

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Absolutely, Do you still have 2v heads? The port surfaces don't match and create all kinds of air/fuel turbulance issues. The 2v heads have an intake port opening of 2.02 X1.65 where the 4v head and intake has an intake opening of 2.5 X 1.75 inches. So there is a damming effect of fuel and air going into the 2v head from the 4v intake.

2rr7aiv.png

 

Just cruising along minding our own business when BAM!!! The LAWS show up.

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That sounds like the cause right there. 4V intake on a 2V head. Get yourself a edlebrock performer asap. Try to find one cheap or on ebay.

2011 Buick Regal

2010 Ford F150 Screw

2002 Triumph Trophy

1973 Dodge Challenger RT

1972 Mustang Grande Coupe

1954 Ford F100

 

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I previously had the stock Ford 2V intake and a 2V Autolite carb, then I switched to that Weiand 4V intake that is built to be mounted on 2V heads :

http://www.summitracing.com/parts/WND-7516/Application/?prefilter=1

And installed the Holley four barrel carb.

It fitted all right, no leak or anything. I don't think that this is the problem because I still had those "bangs" using the stock intake and carb.

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O.K. you do have the correct 4 BARREL intake (not 4v)for your 2V heads. Whew! When you take your old headers off, look in the exhaust ports of the heads and see how much carbon build up is around your exhaust valve stems.

Be very specific when you buy your headers as to your transmission, c4-c6 ect. or they won't fit into the engine compartment.

2rr7aiv.png

 

Just cruising along minding our own business when BAM!!! The LAWS show up.

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Compression test. Cheap, easy to do.

'Mike'

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

 

Pics of modifications included in: [button=http://www.7173mustangs.com/thread-1973-convertible--3335]My Garage[/button]

 

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Based on the information you posted, I tend to think your problem may be the "headers" you currently have.

 

Keep in mind that your car did not come with headers; you would have had exhaust manifolds from the factory. It is possible that your replacement headers (if that is what you have) are "leaking" and that could contribute to "popping" and/or backfires through the exhaust.

 

Just a thought!

 

BT

Do the RIGHT thing.

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I was having the same deceleration popping on my 351C, and it turned out to be a burnt exhaust valve. How many miles are on the motor? Have you done a cylinder leakdown test to make sure the valvetrain is healthy?

 

---

71 Mach 1, bright red/black, 351C, FMX

67 Fairlane GT, black/parchment, 390, 4sp

 

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Based on the information you posted, I tend to think your problem may be the "headers" you currently have.

 

Keep in mind that your car did not come with headers; you would have had exhaust manifolds from the factory. It is possible that your replacement headers (if that is what you have) are "leaking" and that could contribute to "popping" and/or backfires through the exhaust.

 

Just a thought!

 

BT

+1 Couild be sucking cold air back into the exhaust system.

 

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Timing that is too far retarded can cause this too. Back in the day it was cool to have the car backfire or pop when decelerating.

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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Sounds like a timing issue to me too. Does it only backfire on deceleration? What is your idle like. It may be a carb adjustment, try a richer mix and see.

2011 Buick Regal

2010 Ford F150 Screw

2002 Triumph Trophy

1973 Dodge Challenger RT

1972 Mustang Grande Coupe

1954 Ford F100

 

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