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Old 10-19-2018, 01:57 PM   #41
Cape Codder
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Default Re: Terriable backfiring

Sooo....I have checked a couple things that were suggested. Started the engine and ran for several minutes to warm it up. Grounded each plug and each showed a difference in idle. Don't understand BUT I will be putting in NEW Champion W18 plugs. I think they are a colder running plug but....... I then took the car around town for perhaps 10 miles and it backfired a couple of times, and several times actually quite. At the end of driving the engine was really smooth. I also removed the plugs and did a compression check. 1 - 2 - 4 shoed 60 pounds BUT 3 showed at best 48 pounds. Do I have a head gasket problem? Always thought that a 10% variance in pressure was acceptable but this is 20%.
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Old 10-19-2018, 02:18 PM   #42
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Default Re: Terriable backfiring

I wonder if you have a leaking exhaust valve that is letting raw gas into
the exhaust system when the piston is coming up on compression. I would do a leak down test to see where the compression on #3 is going.



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Old 10-19-2018, 02:25 PM   #43
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Default Re: Terriable backfiring

a 12 LB difference when all other 3 are at 60 looks like a problem. Could be head gasket, could also be a valve issue. Not sure if it would cause backfiring, others may know.


Sticky valve, carbon under valve, bent valve, etc.


Take the plugs out.


Crank the motor with ignition off or hand crank and you may be able to view the valve operation through the spark plug hole


Or take the valve cover off, crank engine and view valve operation


Or use a borescope through the spark plug hole.


Or perform a leak down test.


To see if you have a head gasket leak into the coolant - look for bubbles in the radiator coolant with the engine running, also load the engine down and recheck. As a final check you can purchase a NAPA Block Test and test the coolant for exhaust gas (the test kit is about $45).
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Old 10-19-2018, 04:53 PM   #44
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Default Re: Terriable backfiring

My stock engine has only 50 PSI on all 4 cylinders, but runs strong and smooth.
10 years ago all the bad backfiring was caused by poor contacts in the repro switch.
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Old 10-19-2018, 05:33 PM   #45
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Default Re: Terriable backfiring

Tom Wesenberg like I said in a previous post I installed the jumper for the switch and it still BACKFIRED! So I think the switch is not an issue. Thank You!

Bob C and 30 Closed Cab PU I plan on doing a leak down test tomorrow. I just have to read how it is done. I had done one years ago but will brush up with the instructions. Thanks to your both!!
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Old 10-19-2018, 05:45 PM   #46
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Default Re: Terriable backfiring

Donít discount a failed manifold gasket.
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Old 10-19-2018, 06:45 PM   #47
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Default Re: Terriable backfiring

JacksonIII it was just replaced and then checked with a butane torch that was suggested earlier.
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Old 10-20-2018, 09:46 AM   #48
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Default Re: Terriable backfiring

Take the car out and drive it hard for 20 or 30 miles, and then see how it runs.
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Old 10-20-2018, 02:40 PM   #49
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Default Re: Terriable backfiring

There are many causes to a backing fire on the Model A. The two I recently discovered was a loose coil cable. Be sure the cable from the coil to the distributor is seated tight in the coil. if it is loose it can arc causing a backfire. the second was a badly clogged carburetor. The gas had gone bad for sitting too long and gummed everything up. I cleaned the carburetor and replaced the gasket where it bolts to the intake to make sure I had no leaks. That seamed to solve the problem for me.
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Old 10-20-2018, 04:56 PM   #50
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Default Re: Terriable backfiring

kathy Thanks for responding. Well, I believe I did that a month ago when I went to Lake George, N.Y. and back! 650-700 miles!
gibbdoHonestly I have changed the COIL WIRE but it didn't seem to seat as well as the old. I attributed that to being new. I will have to revisit my coil wire. I have yet to take the carburetor off because at this point I don't believe I have a fuel problem i.e. dirt in the tank/carburetor because my glass bowl filter is as clear as can be. I would also like to thank you for responding.
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Old 10-20-2018, 06:17 PM   #51
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Default Re: Terriable backfiring

All these things tried and you HAVEN"T CHECKED FOR FUEL FLOW?! You're assuming because the tank looked nice and clean a while ago and the sed bowl was clean, no problemo--right?


A single chunkie can cause your problem.

First, disconnect fuel line at carb and observe fuel stream onto ground or in a can. Stream should look like that of a young man proudly writing his name in the snow; not an old man laboring to keep his shoes dry.

IF stream is good, try another carb OR perform the carburetor Heimlich: momentarily pop palm of hand over carb intake while simultaneously racing motor. release throttle and remove hand before engine dies. Engine vacuum will suck any little poopies that may be blocking carb jets on thru.

Last edited by steve s; 10-20-2018 at 06:27 PM.
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Old 10-20-2018, 08:07 PM   #52
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Default Re: Terriable backfiring

If you happen to have access to another carb, you may replace the one you have on your rig and try another. Also make sure your carb gasket between the carb and intake manifold is not leaking. I had similar problem as you and when I used a different carb, problem went away. I am no expert on things but this worked for me. Or it may have been a leaky gasket between the carb and manifold. I found this gasket "wet" when I took it off.


I also put some MMO in the gas. I thought I may have a stuck valve and the reason for the MMO.


So don't know what solved the problem.
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Old 10-23-2018, 05:48 PM   #53
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Default Re: Terriable backfiring

steve s - Well you will be happy because I checked for fuel flow!!! It was fine. I also checked the filter in the front of the carburetor. Nothing there either. I'm on another mission trying to get head off but when that is done I will be opening the carburetor up.

After doing a compression check and finding #3 was low I did a leak down test and I could hear air coming from #4 but when I reversed the process the air coming from #3 was not as great as what came from #4. My conclusion is the head gasket is blown so I am trying to remove the head. I don't think it has anything to do with the backfiring.

BUT my question is if the head gasket is blown why did #4 show 60lb. while #3 had only 48lb.
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Old 10-24-2018, 09:29 AM   #54
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Default Re: Terriable backfiring

how can an electrical issue cause a backfire?
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Old 10-24-2018, 11:21 AM   #55
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Default Re: Terriable backfiring

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Originally Posted by Cape Codder View Post
steve s - Well you will be happy because I checked for fuel flow!!! It was fine. I also checked the filter in the front of the carburetor. Nothing there either. I'm on another mission trying to get head off but when that is done I will be opening the carburetor up.

After doing a compression check and finding #3 was low I did a leak down test and I could hear air coming from #4 but when I reversed the process the air coming from #3 was not as great as what came from #4. My conclusion is the head gasket is blown so I am trying to remove the head. I don't think it has anything to do with the backfiring.

BUT my question is if the head gasket is blown why did #4 show 60lb. while #3 had only 48lb.
Were you able to tell where the air was leaking? Intake Manifold, Exhaust Manifold, Crankcase, etc? - this info would help with diagnosing the issue.


Sorry did not read this thoroughly enough - seems you saying the leak is between cylinders. Is it possible with a slight leak the gap/break in the gasket opens, but in the other direction moves back and provides a seal? Just a guess, do not know if this is possible.

Last edited by 30 Closed Cab PU; 10-24-2018 at 11:29 AM.
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Old 10-24-2018, 11:24 AM   #56
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Default Re: Terriable backfiring

I'm not sure I saw a response to JR1967's comment about a plugged exhaust

I had a very similar condition that you have - and I did the exact same things you did to find the problem. In the end, it was baffles in the later style muffler had collapsed and blocked the exhaust. At idle the pressure is low enough that it could get through the small gaps around the blockage, but at higher RPM, the pressure was too great and caused backfiring.

Here's a link to my discussion thread:

https://www.fordbarn.com/forum/showt...ferrerid=17981

Assuming you exhaust system is welded one piece from manifold to tailpipe, the only way to test it is to disconnect the exhaust pipe from the manifold. That of course has some risks, as any backfire is occurring right in front of the carburetor. And honestly, with NO exhaust back pressure, you may well get some backfiring when you let off the gas. But, when I tested it, I increased idle up as high as I felt comfortable, and had no backfire (that is, until I released the throttle and it felt back to idle - but that was minor).

Good luck! And keep a fire extinguisher handy!
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Old 10-24-2018, 06:11 PM   #57
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Default Re: Terriable backfiring

Good afternoon...If you remove the muffler and tail pipe system, and shake it on the driveway...if it rattles than you can be sure that the baffles are loose. You can also shoot compressed air through it and see what happens. Most of us did not think of the muffler...Ernie in Arizona
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Old 10-24-2018, 09:48 PM   #58
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Default Re: Terriable backfiring

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Originally Posted by daren007 View Post
how can an electrical issue cause a backfire?
Easy, if the spark is momentarily interrupted, raw air/fuel mixture gets into the exhaust. Then, when ignition is restored, and fire goes into the exhaust, boom, as that unburned mix explodes. Anything that causes a misfire can cause a backfire.

Don't believe me? Drive your A along and turn off the key for a second or so, then turn it back on. Boom. I once destroyed a muffler when scaring my neighbor. He had the last laugh on that one, but it occurred when I was in high school, back in the '50s and I have since gotten a tiny bit smarter.
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Old 10-24-2018, 10:49 PM   #59
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Default Re: Terriable backfiring

30 Closed Cab PU Thanks for your response. I was thinking the same thing about the gasket BUT I have to get the head off before I can report how it looks.

saintjoelarry Well I haven't taken the muffler off yet. It is an Aries Stainless Steel (put on new 11,000+ miles ago) although I'm sure that doesn't matter. Thanks for your response also.

Ernie Vitucci Thank you also for the comment and suggestion.

Corley Thanks for your response and also a great explanation!!

I will make comments when I get the head off. I also plan on checking the valve clearances while I'm at it. Then I will see if either the blown gasket? or the valves being out of adjustment caused the backfiring which I'm not really convinced it is. If not I will have to continue on with the trouble shooting.
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Old 10-25-2018, 08:32 AM   #60
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: ... momentarily pop palm of hand over carb intake while simultaneously racing motor. release throttle and remove hand before engine dies. Engine vacuum will suck any little poopies that may be blocking carb jets on thru.

If it's not too late (head already off) do me a personal favor and try this. It will only take 15 seconds. Probably prudent to wear a glove on your left hand, although I've done it a hundred times and never had a problem.
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