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EFV-8 Club Forum / General Ford Discussion / Backfiring

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Discussion Topic: Backfiring

Dustbowl

03-24-2018 @ 6:01 PM

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Posts: 74
Joined: Feb 2011

My 36 flathead (one 97 carb) has dual exhausts and straight through mufflers. When they are warm, I get a lot of sharp backfires in the exhaust when de-accelerating. What causes this?

I sent this exact same question to this forum 7 years ago and received a few answers, all of which suggested an air leak in the intake system, such as via:
-worn throttle shaft bushing in the carb,
-cracked vacuum hose,
-leak in vacuum line, carb or intake manifold gasket.

I pursued all these ideas but could not eliminate the backfiring. So I have lived with it for 7 more years (and kind of like it).

Now, however, I am replacing the "smitty" type mufflers with conventional ones and I am concerned that the backfiring explosions may damage the new muffler baffles.

In the search for a solution I have become to doubt the "air leak" theories for two reasons:'
-even with an air leak, why doesn't the fuel/air mix burn in the cylinder instead of passing thru and burning in the exhaust?
-and what is it about old fords that give them the propensity to backfire?

My tentative answer to these questions is that the distributor vacuum brake retards the spark too much on de-acceleration, causing a misfire in the cylinder - and thus leaving the air/fuel mix to explode when hitting the hot exhaust pipe.

I have not tested this theory by adjusting the vacuum brake yet. I ask for your comments/suggestions again before making any changes.

Incidentally, my engine runs very well, has good power and idles smooth.

Apologies for long story but please reply.

BUCKAWHO

03-25-2018 @ 4:43 AM

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I would suspect problems with the distributor, or the timing. Also possibly too rich fuel mixture. Vacuum leaks not so much. BT

Yokomo99

03-25-2018 @ 7:04 AM

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All engines do that noise and loud pops out of the exhaust on deceleration. I don't call it backfiring as it doesn't come out of the carb. It is caused by an overly rich mixture actually igniting in the manifolds and exhaust piping. The Volkswagen Bug engine tried to suppress this noise by retarding the timing under high manifold vacuum conditions with limited success. When you have stock type mufflers on you don't hear it because the muffler muffles it. The straight through design mufflers ( should "REALLY" be called un-mufflers) don't muffle the pops so you can hear them. The backpressure in the stock exhaust system also suppresses the popping.

Be sure to check out my YouTube pages at:

https://www.youtube.com/user/MatthewBergin1

https://www.youtube.com/channel/UCJCdOfmXtM8xO9nXeQ-iSQQ

JohnA

03-25-2018 @ 11:26 AM

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Good answer.Johna

sarahcecelia

03-25-2018 @ 1:43 PM

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How about late timing??? That cold cause it to fire on exhaust. When I was a kid, 18, I retarded my timing a little on my 1950 deluxe V8 so it would "pop. pop, pop" on deceleration. I thought that sounded COOL!!!

Regards, Steve Lee

Dustbowl

03-25-2018 @ 1:57 PM

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I get somewhat conflicting views from the above posts.

Sarahcelia concurs with my view that the popping on deceleration is caused by an excessively retarded spark.

Yomomo99 says it is due to overly rich fuel mix but more importantly, he thinks it happens in all cars and the mufflers dampen the sound.

If I accept the latter view, I don't have to worry that the popping will damage my new mufflers. Can I be confident he is right?

Yokomo99

03-25-2018 @ 4:32 PM

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Runs very well has good power and idles smooth is not a description of an engine with late ignition timing.

Dustbowl Every engine I have installed "Cherry Bomb" style straight through mufflers on has popped and crackled on deceleration. I also liked the sound a lot when I was younger. My son's Dodge Dakota with a computer controlled fuel injection engine and a straight through muffler does the same thing. If these so called back fires are really loud I mean like almost wake the dead. Then I would worry but from your description I don't think that is the case.

Matt

Be sure to check out my YouTube pages at:

https://www.youtube.com/user/MatthewBergin1

https://www.youtube.com/channel/UCJCdOfmXtM8xO9nXeQ-iSQQ

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