Backfiring

[ModelAMitch]

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

[mcgarrett]

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

[cavalierhome]

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.

[aermotor]

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?

[ModelAMitch]

Originally Posted by mcgarrett

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.

Originally Posted by cavalierhome

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

Originally Posted by aermotor

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.

[BillCNC]

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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
Bill

[Marshall V. Daut]

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

[1crosscut]

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.

[ModelAMitch]

Originally Posted by BillCNC

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.

Originally Posted by Marshall V. Daut

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...

Originally Posted by 1crosscut

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.

[rotorwrench]

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.

[Brentwood Bob]

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.

[Rob Doe]

Originally Posted by aermotor

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.

[Rob Doe]

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)

[1crosscut]

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.

[ModelAMitch]

Originally Posted by 1crosscut

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!

[rotorwrench]

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.

[ModelAMitch]

Originally Posted by rotorwrench

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..

[wwirz]

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.

[bobbader]

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.

[ModelAMitch]

Originally Posted by bobbader

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.