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Old 09-15-2022, 07:26 PM   #1
ModelAMitch
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Default Backfiring

Hey guys, question here about my car backfiring.. I haven’t driven my A too much since owning it and my timing gear went. I replaced that and did a bunch of other things. Now it’s running and I’ve driven it twice since my repairs and it’s backfiring, mostly on downshifts but sometimes during acceleration as well. It seems like it’s happening in the engine bay and not out the tail pipe. It sounds like it has an exhaust leak so I believe my manifolds need to be machined and new gaskets probably? If my manifolds are leaking this could cause the car to backfire right? Im mainly concerned because I was told that backfiring could have caused my timing gear to fail prematurely? I want to drive my car now that it’s back together and running, but I definitely don’t want to be doing another timing gear. I plan on addressing this manifold issue over the winter but I’d really like to drive it for a few weeks before putting it away for the winter, then I’ll take more stuff apart and address more issues. Do you guys think any harm is being done to the timing gear with the car back firing every now and then? And could there be another reason for the odd backfire other then the manifolds leaking?
Thanks Fordbarn
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Old 09-15-2022, 07:57 PM   #2
mcgarrett
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Default Re: Backfiring

Speaking from my own experience, my car had some fuel flow problems and would sometime starve for fuel which would cause it to backfire due to intermittent fuel flow while driving above 30 mph. Once I cleared the blockage and got the fuel flowing good again the backfire went away and the car ran normally. Backfiring can also be due to a failing condenser, broken wire under the point plate, etc. If your timing gear is new, I'd begin to check the items mentioned above along with checking your timing to be sure nothing has changed there
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Old 09-16-2022, 05:22 AM   #3
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Default Re: Backfiring

Backfire can be cause by lack of fuel. check the entire fuel run from inside the tank to the carburetor, and then inside the carb at the filter and the float.

I had that happen because an extra gasket was stuck inside the small glass bowl filter.

If you get too many big backfires then your muffler will split open at the top where the seems are, and then you will get perpetual backfires. I had to replace my muffler because of backfires, but that turned out to be a good thing in the end.

If you are backfiring when coating down hill check your timing, you may be way advanced.

But really look for a leak in the exhaust. With the car running put you hand just above the muffler (if you have a heat shield along side the space), do you feel exhaust air? Replace the muffler.

These all happened to me.
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Old 09-16-2022, 07:31 AM   #4
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Default Re: Backfiring

I also had backfiring on deceleration and found that 1 full turn CCW on the GAV fixed it, runs in all other modes well. Forgot - does in or out lean or enrich? Does 1 full turn out sound correct? Or, do I have a jet issue?

Last edited by aermotor; 09-16-2022 at 07:44 AM.
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Old 09-16-2022, 08:25 AM   #5
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Default Re: Backfiring

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Originally Posted by mcgarrett View Post
Speaking from my own experience, my car had some fuel flow problems and would sometime starve for fuel which would cause it to backfire due to intermittent fuel flow while driving above 30 mph. Once I cleared the blockage and got the fuel flowing good again the backfire went away and the car ran normally. Backfiring can also be due to a failing condenser, broken wire under the point plate, etc. If your timing gear is new, I'd begin to check the items mentioned above along with checking your timing to be sure nothing has changed there
Ive had my tank off to clean the rust out of it, have cleaned my carb recently and I have an inline fuel filter I change frequently whenever it gets a bit of sediment in it. I don't think Ive reached 30mph since changing the timing gear haha. But I have had my distributor apart and have not changed any of those parts before. Im hoping to make it to winter time before rebuilding the distributor. But may need to earlier if backfiring persists.

Quote:
Originally Posted by cavalierhome View Post
Backfire can be cause by lack of fuel. check the entire fuel run from inside the tank to the carburetor, and then inside the carb at the filter and the float.

I had that happen because an extra gasket was stuck inside the small glass bowl filter.

If you get too many big backfires then your muffler will split open at the top where the seems are, and then you will get perpetual backfires. I had to replace my muffler because of backfires, but that turned out to be a good thing in the end.

If you are backfiring when coating down hill check your timing, you may be way advanced.

But really look for a leak in the exhaust. With the car running put you hand just above the muffler (if you have a heat shield along side the space), do you feel exhaust air? Replace the muffler.

These all happened to me.
Thank you for the advice, Im fairly certain the backfirings are happening from under the hood, but Ill let the car idle tonight and get my hands under and around the muffler to feel for exhaust leaks.

I did notice backfiring when Im off the gas and coasting down a hill, it could definitely be that my timing is too advance during this time. I will pay attention to where my lever is next time Im doing this. Thank you

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Originally Posted by aermotor View Post
I also had backfiring on deceleration and found that 1 full turn CCW on the GAV fixed it, runs in all other modes well. Forgot - does in or out lean or enrich? Does 1 full turn out sound correct? Or, do I have a jet issue?
CCW will enrich the mixture (add more fuel) I have my GAV almost a half turn out. I havent messed with it while Im driving, but while the cars idling if I turn it either way it starts to run funky and doesnt like it. Atleast at idle I think Ive found the sweet spot, although everyone says each Model A prefers a different GAV adjustment.
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Old 09-16-2022, 10:37 AM   #6
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Default Re: Backfiring

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99.9% of the time

Backfire out of the carburetor, ... Your timing is off.
Backfire out of the tail pipe, ... Your fuel mixture is to lean.

The other .9% is your wire periodically shorting out inside the armored ignition cable.

Regards
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Old 09-16-2022, 10:41 AM   #7
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Default Re: Backfiring

In addition to what the guys have suggested so far, check the exhaust manifold-to-muffler clamp orientation, specifically the gap between the two clamp halves. If the manifold and muffler pipe are not 100% sealed at this juncture, exhaust can blow past and be sucked into the carburetor's throat. That will cause popping and backfiring, especially during deceleration. Loosen the clamp nuts and rotate the clamp so that the gap between halves is not directly facing the carb's throat. This is an overlooked cause of popping/backfiring that is hard to diagnose unless there are big, black ugly splotches of exhaust soot near this juncture. This is not always the case with leakage in this area.
Marshall

Last edited by Marshall V. Daut; 09-16-2022 at 11:55 AM.
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Old 09-16-2022, 12:07 PM   #8
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Default Re: Backfiring

Your inline fuel filter could very well be the cause of your problem. If you have had your tank cleaned and have installed a pencil filter in the top of your fuel shut off valve then you should be good.

Model A's are not necessarily fond of the inline fuel filters.
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Old 09-16-2022, 01:32 PM   #9
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Default Re: Backfiring

Quote:
Originally Posted by BillCNC View Post
99.9% of the time

Backfire out of the carburetor, ... Your timing is off.
Backfire out of the tail pipe, ... Your fuel mixture is to lean.

The other .9% is your wire periodically shorting out inside the armored ignition cable.

Regards
Bill

Interesting, thanks Ill look into this. Ill play with the GAV next time Im driving. Also, my ignition cable was electrical taped into the distributor from the previous owner. Sounds like this could be a problem aswell.


Quote:
Originally Posted by Marshall V. Daut View Post
In addition to what the guys have suggested so far, check the exhaust manifold-to-muffler clamp orientation, specifically the gap between the two clamp halves. If the manifold and muffler pipe are not 100% sealed at this juncture, exhaust can blow past and be sucked into the carburetor's throat. That will cause popping and backfiring, especially during deceleration. Loosen the clamp nuts and rotate the clamp so that the gap between halves is not directly facing the carb's throat. This is an overlooked cause of popping/backfiring that is hard to diagnose unless there are big, black ugly splotches of exhaust soot near this juncture. This is not always the case with leakage in this area.
Marshall
I have a bit of a leak at the exhaust clamp at the bottom of the header, I removed it and muffler cemented the Sh** out of it, but still has a slight leak. I didnt think about the exhaust gases getting sucked back into the carb...

Quote:
Originally Posted by 1crosscut View Post
Your inline fuel filter could very well be the cause of your problem. If you have had your tank cleaned and have installed a pencil filter in the top of your fuel shut off valve then you should be good.

Model A's are not necessarily fond of the inline fuel filters.
The reason for the inline filter, I cleaned the rust in the tank out myself and did the best I could but theres still small bits I can see if I look into the filler neck with a flashlight, but its much much better then before. I did put the pencil filter on the fuel valve. But the filter eventually gets dirty with rusty coloured sediment in it. No chunks of course, but theres sometimes a little bit of what looks like sand that settles in the bottom of the filter. So Im hesitant to remove the inline filter just yet.
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Old 09-16-2022, 01:43 PM   #10
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Default Re: Backfiring

Back fire is through the carburetor during a lean mixture situation. There is a flow problem somewhere. It could be on the supply side or internal to the carburetor.

After fire is the combustion in the tail pipe or muffler. Sticky or burned exhaust valves are the common cause of that.
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Old 09-16-2022, 01:56 PM   #11
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Default Re: Backfiring

With a rusty fuel supply I would reflush the tank, put a magnet in the glass sentiment bowl. Fix the exhaust leak, and clean the carb.
Do one thing at a time.
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Old 09-16-2022, 01:56 PM   #12
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Default Re: Backfiring

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Originally Posted by aermotor View Post
I also had backfiring on deceleration and found that 1 full turn CCW on the GAV fixed it, runs in all other modes well. Forgot - does in or out lean or enrich? Does 1 full turn out sound correct? Or, do I have a jet issue?
CCW on the GAV enriches the mixture. 1 full turn CCW from Closed is pretty much all the extra fuel you can get on a Zenith. Is your carb a Zenith?

IMHO, 1 full turn CCW on the GAV is too much. Here's some ideas, check for air leaks at the throttle plate shaft, the carb to manifold, manifold to block, wiper connection, set your base idle rpm / Idle Air Adjustment Valve, clean the carb passages.
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Last edited by Rob Doe; 09-17-2022 at 04:35 AM.
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Old 09-16-2022, 02:06 PM   #13
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Default Re: Backfiring

Back fire through the exhaust is sometimes called after fire and is often a rich mixture allowing raw fuel to make it into the exhaust. This happens sometimes as the car is turned off and, if I remember correctly, when you let off the gas to coast.

That said, an exhaust leak can make popping sounds too as Mr. Daut has said. I learned from his post about the exhaust contaminating the intake air horn of the carb.

On radial aircraft engines, exhaust leaks can make popping sounds due to rapid expansion / contraction of air moving in or out of the exhaust leak itself. (So I was taught)
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Last edited by Rob Doe; 09-16-2022 at 02:39 PM.
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Old 09-16-2022, 04:02 PM   #14
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Default Re: Backfiring

Just remove the inline filter for a short period to see if the issue resolves. If it does you've found the problem. If it doesn't put it back on.
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Old 09-16-2022, 04:28 PM   #15
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Default Re: Backfiring

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Originally Posted by 1crosscut View Post
Just remove the inline filter for a short period to see if the issue resolves. If it does you've found the problem. If it doesn't put it back on.
I will do this first and see if anything changes. Thank you!
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Old 09-16-2022, 06:02 PM   #16
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Default Re: Backfiring

You should have a good flow at the carburetor inlet fitting with the line disconnected and the fuel valve on. If you don't them it's on the supply side. If flow is good then it's in the carburetor. Crud can block the float valve or the jets but something is getting low flow.
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Old 09-16-2022, 06:06 PM   #17
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You should have a good flow at the carburetor inlet fitting with the line disconnected and the fuel valve on. If you don't them it's on the supply side. If flow is good then it's in the carburetor. Crud can block the float valve or the jets but something is getting low flow.
Thanks rotorwrench.
With the fuel line off the carb and gas valve turned on gas is flying out of there pretty quickly. I’d say there is sufficient fuel flow. So I guess maybe I’ll take the carb off and give it a good clean again. I’m getting really good at taking this carb apart..
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Old 09-16-2022, 07:51 PM   #18
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Default Re: Backfiring

I had backfiring and sputter. Turned out to be the condenser. I could not keep condensers installed for more than 50 miles until I had to change it for backfiring issues. I went to the modern upper and lower plate design in the distributor and I never had any problems since. Improved the reliability of the car immensely.....no issues.
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Old 09-16-2022, 08:04 PM   #19
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Default Re: Backfiring

I notice nobody has asked whether you’re using a Zenith carb, a Tillotson carb, a Marvel carb. The Tillotsons and Marvels, being pot metal, have a tendency toward at the mounting flange, causing vacuum leaks, which will definitely have an effect on overall running.

And, while having the GAV adjustment at the choke rod open a full turn is usually way too much for a Zenith, 1/2 to 3/4 turn open for Tillotsons is not that strange.
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Old 09-17-2022, 04:31 AM   #20
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I notice nobody has asked whether you’re using a Zenith carb, a Tillotson carb, a Marvel carb. The Tillotsons and Marvels, being pot metal, have a tendency toward at the mounting flange, causing vacuum leaks, which will definitely have an effect on overall running.

And, while having the GAV adjustment at the choke rod open a full turn is usually way too much for a Zenith, 1/2 to 3/4 turn open for Tillotsons is not that strange.
I have a marvel carb with the GAV turned out almost a half turn.
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Old 09-17-2022, 04:39 AM   #21
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Default Re: Backfiring

Thanks for your post bobbader. I edited my post in #12 to indicate Zenith for those that may read thru this thread at a later time.
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Old 09-17-2022, 04:54 AM   #22
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Default Re: Backfiring

Quote:
Originally Posted by ModelAMitch View Post
Interesting, thanks Ill look into this. Ill play with the GAV next time Im driving. Also, my ignition cable was electrical taped into the distributor from the previous owner. Sounds like this could be a problem aswell.




I have a bit of a leak at the exhaust clamp at the bottom of the header, I removed it and muffler cemented the Sh** out of it, but still has a slight leak. I didnt think about the exhaust gases getting sucked back into the carb...



The reason for the inline filter, I cleaned the rust in the tank out myself and did the best I could but theres still small bits I can see if I look into the filler neck with a flashlight, but its much much better then before. I did put the pencil filter on the fuel valve. But the filter eventually gets dirty with rusty coloured sediment in it. No chunks of course, but theres sometimes a little bit of what looks like sand that settles in the bottom of the filter. So Im hesitant to remove the inline filter just yet.


Take the bowl filter apart. Then look up inside the top part. Is there a SECOND gasket (the small gasket at the tip of hew insert filter) accidentally left there when you changed the filter? If so it is cutting off your flow.

I had that exact same thing happen to me. I took everything apart and didn't see it. It was only when I was using compressed air to clear all passages out that the stupid thing popped out.

The car would run ok sometimes and sometimes it would backfire a lot, on and off. It took me awhile to figure that one out.
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Old 09-17-2022, 05:17 AM   #23
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Default Re: Backfiring

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Originally Posted by cavalierhome View Post
Take the bowl filter apart. Then look up inside the top part. Is there a SECOND gasket (the small gasket at the tip of hew insert filter) accidentally left there when you changed the filter? If so it is cutting off your flow.

I had that exact same thing happen to me. I took everything apart and didn't see it. It was only when I was using compressed air to clear all passages out that the stupid thing popped out.

The car would run ok sometimes and sometimes it would backfire a lot, on and off. It took me awhile to figure that one out.
Very interesting.. I’ll certainly give that a look once I get the carb off again thank you
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Old 09-17-2022, 07:02 AM   #24
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Default Re: Backfiring

Well, now we know what kind of carb. I have no first hand experience with Marvel carbs, but, my advice would be to play with the choke rod adjustment ............. open or close a bit to see if things change. Assuming the carb is in good condition, this might tell you it’s just a matter of finding the sweet spot where it runs best.

Of course, there have been many other good suggestions made here and all of them are viable. Take heed .......... but remember before you start modifying (having the manifolds planed, etc) try CALMLY going through each suggestion to see if things change before you move on to the next suggestion. The Process of Elimination is always your best friend. Make a list and go through the possibilities “one by one” before you make any bold moves to try and cure 10 things at once.

Good luck.
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Old 09-17-2022, 05:39 PM   #25
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Default Re: Backfiring

The armored cable! I had a nick in the cable between the firewall and the dash. Something that wasn't normally visible.I was having problems of the car running, not running, running. You get the idea. There were times I'd back it out of the garage to let it warm up and the it would die. All due to the nick in the cable. I found it by pulling off the dash to see if I could find anything that was causing he intermittent running. As i was fooling around with the dash, I saw a few small sparks. Long story short, I pulled all that old wiring from the dash and replaced it. The Shielded cable nick was repaired and the car now runs quite well! All those problems were because of a darn nick in that cable!
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Old 09-17-2022, 06:57 PM   #26
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Quote:
Originally Posted by Neshkoro View Post
The armored cable! I had a nick in the cable between the firewall and the dash. Something that wasn't normally visible.I was having problems of the car running, not running, running. You get the idea. There were times I'd back it out of the garage to let it warm up and the it would die. All due to the nick in the cable. I found it by pulling off the dash to see if I could find anything that was causing he intermittent running. As i was fooling around with the dash, I saw a few small sparks. Long story short, I pulled all that old wiring from the dash and replaced it. The Shielded cable nick was repaired and the car now runs quite well! All those problems were because of a darn nick in that cable!
Wow.. lucky nothing worse happened! It looks like my ignition cable has seen better days but the car has no troubles starting and running, it just backfires occasionally while driving. However, the ignition cable is on my to do list for this winter.
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