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Old 04-02-2019, 06:53 PM   #1
billmessmer
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Default backfire.

What are some common causes of backfire?
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Old 04-02-2019, 07:00 PM   #2
Jim/TX/GA
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Default Re: backfire.

Loud explosion backfire out of muffler/exhaust is caused by having a lot of unburned fuel collecting in the exhaust system.

This is usually because of faulty ignition. So the plugs are not firing (burning the fuel) and it goes into the exhaust.

So look at: bad condenser, ignition points in bad condition, point gap too small, bad connectors from distributor cap to plugs, fouled plugs.

An ignition switch going bad can do this, too. Or bad connections at the coil.

HTH.
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Old 04-02-2019, 10:08 PM   #3
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Default Re: backfire.

Back firing is through the carburetor. After firing is through the exhaust.
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Old 04-03-2019, 07:08 AM   #4
Curtis in MA
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Default Re: backfire.

I have never heard that term " After Firing".
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Old 04-03-2019, 07:29 AM   #5
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Default Re: backfire.

Recently my 30 while on a outing was running fine and then started to bog down (no power) and backfire. Fuel starvation. Loosened gas cap, heard a tell tale "poof" sound. Fuel vent in gas cap was plugged. Replaced cap "loosely" and finished the trip. Used a small drill bit to clear the vent. Chap
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Old 04-03-2019, 07:40 AM   #6
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Default Re: backfire.

I have a gas cap that we use with a small hole drilled in the very center of the cap.

One move potential problem solved.
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Old 04-03-2019, 08:50 AM   #7
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Default Re: backfire.

Leaky manifold gasket can do it.
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Old 04-03-2019, 11:06 AM   #8
Bob C
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Default Re: backfire.

I never heard of after fire either, learned something.
https://en.wikipedia.org/wiki/Back-fire


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Old 04-03-2019, 01:37 PM   #9
Curtis in MA
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Default Re: backfire.

The first sentence of that seems to say that it can also be called backfire.
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Old 04-03-2019, 01:39 PM   #10
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Default Re: backfire.

Quote:
Originally Posted by chap52 View Post
Recently my 30 while on a outing was running fine and then started to bog down (no power) and backfire. Fuel starvation. Loosened gas cap, heard a tell tale "poof" sound. Fuel vent in gas cap was plugged. Replaced cap "loosely" and finished the trip. Used a small drill bit to clear the vent. Chap

Could it have been an 'after fire'?
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Old 04-03-2019, 01:59 PM   #11
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Default Re: backfire.

Years ago when gasoline had lower octane and had less additives it was common for Model A's to need a valve job. The symptom was mild backfiring through the exhaust system when descending a long downhill grade with little acceleration from the accelerator needed. I've had this occur twice on long trips, once in 1947 and again in 1955. It was not unusual in those days and the old time mechanics knw the symptom and the repair needed. Needing the valves ground does not seem to be a problem now or in recent years.
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Old 04-03-2019, 04:28 PM   #12
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Default Re: backfire.

Lot of screwy information.
Question: Does it cough out of the carbuerator air horn, or pop out of the tail pipe???

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Old 04-03-2019, 05:44 PM   #13
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Default Re: backfire.

I recently got a car running and every time I let off on the gas, it would backfire. So I turned the GAV out a couple turns and no more backfiring. This carburetor is running lean. A too lean air fuel mixture for any reason will cause backfire, as stated above.
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Old 04-03-2019, 05:51 PM   #14
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Default Re: backfire.

I seen a bad condenser or a condenser on its way out causing a backfire
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Old 04-03-2019, 05:57 PM   #15
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Default Re: backfire.

Yesterday got my truck back from a Model A shop, turned it off, and within a couple of seconds backfired through he exhaust. Got it running again at idle , GAV set full lean,advance lever up - was not running right. Got out and noticed black smoke out of the exhaust. Backed the idle mixture screw out a couple of turns until it started running better - issue solved.


Exhaust was loading up on unburned gas, and was ignited by the hot exhaust manifold.
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Old 04-03-2019, 07:11 PM   #16
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Default Re: backfire.

Float set too low on zenith carb. can cause back fire when gas petal is released. Lean mixture...
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Old 04-04-2019, 03:35 PM   #17
Jim/TX/GA
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Default Re: backfire.

Have you all noticed that we got 1 question and 15 replies now and the Original Poster has not come back to clarify if he meant he was getting a true backfire out the intake throat of the carburetor or if he meant he was getting explosive backfire out of the tailpipe?

We have provided answers to both scenarios, but we still don't really know for sure what the symptoms are!

Or perhaps he has solved the problem and never came back to tell us he found a loose wire, or replaced the condenser. And we are rambling on about float levels, leaky intakes, etc. No criticism to the people who have answered, I'm sure others appreciate reading the ideas.

But as a courtesy to others, some follow up by the original poster would be nice.

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Old 04-04-2019, 04:53 PM   #18
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Default Re: backfire.

Daren007 has correctly defined both events. Has always been a pet peeve of mine, because it confuses some when trying to understand a problem correctly. Back fire through the carb. after fire through the exhaust. I am not trying to be a jerk, but it confuses some trying to understand. Relates to the original posters Question.
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Old 04-04-2019, 05:00 PM   #19
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Default Re: backfire.

Question is very open ended, we do not even know if he has a problem. Perhaps he is just asking for educational purposes, or has the issue and wants to know all the possibilities.
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Old 04-04-2019, 09:17 PM   #20
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Default Re: backfire.

As a new Lt. in the OFD I rode in the cab of a 1953 ladder truck that had a bad habit of blasting through the tail pipe after descending a long grade. It had a large Hall Scott engine and would shoot a ball of flame six feet or more when it did the "after fire" thing. I had the drivers keep the cab as close to the middle of the street as they could so we wouldn't start a grass fire while returning to the station On the way to the scene it was no problem as the drivers kept their foot in it, but coming home it sounded like cannon fire.
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