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Old 02-22-2011, 11:07 AM   #1
29FordRoadster
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Default Backfiring Model A

Hi All, We have two model A's with the same problem of backfiring. They both idle great, but when increase the speed they start back firing and almost die. I have taken the following parts off and replaced with parts off good running car. The carb, distributor, coil, replaced the plugs and removed the gas cap. Both have had recent valve jobs, and nothing has helped the problem. Does anyone have any suggestions of what we might try to correct this problem. Thanks to all for any help.
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Old 02-22-2011, 11:13 AM   #2
bogdonj
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Default Re: Backfiring Model A

any air leaks around the manifold or exhaust? Did you recheck timing? faulty condenser? and is your spark advance pulled all the way down when running higher speeds? (I know stupid question, but you will be surprised on how many people I catch not doing this) Others will be able to help out for sure with this.. Once you find out the problem, let us know.
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Old 02-22-2011, 11:23 AM   #3
1931 flamingo
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Default Re: Backfiring Model A

Disconnect the fuel line from the carb and see if you have a good fuel flow from the tank. What was disassembled when the valve jobs were done?
Paul in CT
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Old 02-22-2011, 11:44 AM   #4
Tom Wesenberg
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Default Re: Backfiring Model A

Start with the quick easy checks first. Pull the coil wire from the distributor and hold it 1/4" from the head nut to see if you have a good strong blue spark. If not, find and correct that problem first. If you have good spark, then do as Paul suggests to check for good fuel flow to the carb. If the flow is good, then I'd check for dirt in the carb jets or passages.
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Old 02-22-2011, 11:59 AM   #5
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Default Re: Backfiring Model A

Condensor, just a guess.


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Old 02-22-2011, 12:01 PM   #6
allen
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Default Re: Backfiring Model A

If its been sitting up sticking valves
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Old 02-22-2011, 12:49 PM   #7
Ron in Quincy
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Default Re: Backfiring Model A

One of our club members called me yesterday and said the carburetor I rebuilt for him was acting up; did not want to idle smooth, backfired when trying to excellerate.

The first thing I suspected was lean mixture, possible float setting too low, or blocked fuel flow. When I checked the carburetor the bowl was full to correct level, no shortage of fuel and good flow. I removed the carb, the bowl was clean, no blocked passage ways. Rienstalled the carb, turned on the gas and filled up the carb; no leaks. He started the engine and it ran poorly, backfired when trying to excellerate.

I asked what else he had done and he said, checked timing, installed new distributor with V8 points, set the new points.

As I couldn't find a problem with the carb, I opened up the distributor I found the point rubbing block was setting between lobes on the cam but the points were open; when setting on the lobe he had over .045 clearance on the points; readjusted the points to .020 and the engine started, excellerated good.

As usual, 95 percent of problems are electrical, but the rebuilt carb, in the mind of A'er, is always the problem !!!!

Ron
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Old 02-22-2011, 12:56 PM   #8
Steve Wastler
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Default Re: Backfiring Model A

Back in the 80's I had a problem similar to yours, in my 29 roadster. I checked and checked, the problem would not go away. Don't ask me why but, I changed the battery and the problem went away.
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Old 02-22-2011, 01:24 PM   #9
Larry Seemann
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Default Re: Backfiring Model A

Do you have in-line fuel filters? If so, get rid of them. In my experience backfiring generally indicates fuel starvation. It can be from accessory filters, or junk in the gas tank, and a lot of times it comes from the fuel line being pushed too far into the sediment bowl housing. Disconnect your fuel line from the sediment bowl and see how much line extends beyond the ferrule - it should be no more than 1/8". If it's longer saw off all but 1/8" and reinsert it.
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Old 02-22-2011, 07:26 PM   #10
Frank Nemo
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Default Re: Backfiring Model A

There is a lot of solid advise in this thread regarding causes of back firing. Over the years when I experienced backfiring problems I have traced them to two causes. The first was to a defective condensor and the second was a leaking connection between the manifold and the tailpipe.
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Old 02-22-2011, 07:40 PM   #11
29FordRoadster
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Default Re: Backfiring Model A

Thanks to all who have replied to this thread. We have now tried all of the suggested remedies and still have the problem. We thought we might have bad gas, but putting in fresh gas did not help. We really appreciate all replies, and will notify the board if and when a solution is found. We are still very open to any and all further suggestions. Thanks again to all who have tried to help us.
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Old 02-22-2011, 07:46 PM   #12
Peter J
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Default Re: Backfiring Model A

If the same is happening with both, how about water in the gas?? I'ts the time of year for that little gremlin.

Pete
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Old 02-22-2011, 07:48 PM   #13
CarlG
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Default Re: Backfiring Model A

When I was a teenager driving my first Model A, we always thought it was "cute" to start down a hill and retard the spark. If that didn't make it backfire, then one could always turn off the ignition for a second or two and thn back on. I think once I even blew up a muffler that way. ******Kids!*****
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Old 02-22-2011, 08:12 PM   #14
steve s
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Default Re: Backfiring Model A

When you swapped the distributor, did you trade the plastic body and cap too? A friend with a backfiring problem was ready to push his A over a cliff when we tried a different distributor body and fixed problem.

Does the backfiring occur when you simply speed up the idle, or under a load when driving? If the former, it's probably fuel starved; if the latter, it's electrical.

Steve

Last edited by steve s; 02-22-2011 at 08:55 PM.
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