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Old 07-26-2020, 02:51 AM   #1
DRG-ler
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Default Backfire in overrun mode (load change)

Hi folks, a few days / weeks ago I noticed "new" appearances of my 30s Tudor.

My manifold gaskets were burned as a result of a warped manifold, and the last time I drove this manifold I had backfiring in the exhaust (when accelerating ...). Because of the faulty seal at the time, the whole thing was so uncomfortable that I had always stepped on the gas on the way home.

Now I've got a new manifold. This was completely redesigned and planned together with the intake manifold by the engine manufacturer. The whole thing installed with new seals, runs great! I also installed a seal for the rear silencer for the first time. Until then, this was backsing.

I sealed the carburetor flange with Hylomar sealing paste due to the lack of a paper seal. Should also be tight.

Regardless of all the new / restored seals, the "backfires" have unfortunately remained.
It is now not a loud popping, but more like the turbo brawl of modern cars of higher performance classes. However, I would be interested to know where this comes from and whether / how it could be eliminated.

As such, the engine runs super and quietly, pulls through occasionally, and gives no cause for complaint.

Who has tips?

Greetings Andy.
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Old 07-26-2020, 07:23 AM   #2
30 Closed Cab PU
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Default Re: Backfire in overrun mode (load change)

Carb maybe running lean, try giving the GAV another 1/4 turn CCW, then another 1/4 turn CCW. I find that my Gav has to be set at 1/2 turn open, otherwise I get a little popping/cough when decelerating and when upshifting gears.



Possible intake manifold leak. With fan belt off and motor running spay wd40 around the intake manifold and listen for a change in motor running. Not sure from your description above, when changing manifolds supposed to have then surfaced as a pair, also the copper manifold gaskets are recommended. Also try WD40 with motor running around the Carb/Intake manifold seal and listen for motor running change.


Some use a propane torch (unlit) instead of WD40.
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Old 07-26-2020, 08:46 AM   #3
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Default Re: Backfire in overrun mode (load change)

My car had a persistent backfire when decelerating. I increased the fuel with the GAV but that helped little. Turned out the valve guides were shot. Put new valves and guides (and ground the valves and seats of course) and no more backfire.
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Old 07-26-2020, 11:57 AM   #4
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Default Re: Backfire in overrun mode (load change)

Valve guides, ... I don't want to think that far.
However, the problem should be identified by means of a compression test / pressure loss test.

I tried to open GAV further. Even with a full turn, there was no change. So I don't think he's running too lean. But there the spark plug picture should provide information.
Alternatively, I made a lambda sensor adapter that can be installed between the manifold and the rear silencer. It is possible that you will also gain new knowledge in this way.

The thing with the leaky manifold gaskets I find absurd, because I also made sure that the installation was reasonable. Nevertheless, I will check it all again. I will use brake cleaner for this.

Greetings Andy
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Old 07-27-2020, 08:31 AM   #5
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Default Re: Backfire in overrun mode (load change)

Quote:
Originally Posted by DRG-ler View Post
Valve guides, ... I don't want to think that far.
However, the problem should be identified by means of a compression test / pressure loss test.

I tried to open GAV further. Even with a full turn, there was no change. So I don't think he's running too lean. But there the spark plug picture should provide information.
Alternatively, I made a lambda sensor adapter that can be installed between the manifold and the rear silencer. It is possible that you will also gain new knowledge in this way.

The thing with the leaky manifold gaskets I find absurd, because I also made sure that the installation was reasonable. Nevertheless, I will check it all again. I will use brake cleaner for this.

Greetings Andy

I was just trying to throw some easy things at you, hoping that it would get the discussion started. Personally even if testing does not make sense, I like to try it anyway, then I can know/prove it wasn't the issue and can check it off the list of possibilities. I do not like to assume anything when troubleshooting. Seems you are most likely a better mechanic than me and have already performed some additional troubleshooting to narrow the issue down.
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Old 07-27-2020, 08:45 AM   #6
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Default Re: Backfire in overrun mode (load change)

I am not familiar with "turbo brawl", so was probably off on a tangent anyways. If it is more of a whistling, or air type sound, have heard that a carb worn throttle plate shaft
is a cause. If am off base, my apologies.
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Old 07-27-2020, 08:51 AM   #7
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Default Re: Backfire in overrun mode (load change)

After fire in the exhaust can be due to an overly rich fuel mixture or leaky exhaust valves. Sticking valves can also cause enough delay in closing that compression gasses leak into the exhaust. A malfunctioning ignition system can sometimes cause a spark at a time when a valve is open to the exhaust but it usually warns the operator in other ways as well. It can cause missing and popping in the exhaust. The jumping sparks can usually be seen while the engine is running at night.

Lean conditions cause back fire through the carburetor throat. Some folks refer to an after fire condition as back fire but it doesn't fit the definition very well. When the exhaust was mentioned, I could tell the meaning though.
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Old 07-27-2020, 12:15 PM   #8
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Default Re: Backfire in overrun mode (load change)

Quote:
Originally Posted by 30 Closed Cab PU View Post
I was just trying to throw some easy things at you, hoping that it would get the discussion started.
If am off base, my apologies.
Everything okay, I didn't get it wrong. Everything is good. You don't have to apologize for wanting to solve my problems either. The Tip with the valves is not wrong in itself, but it is the most complex and expensive problem, so I don't want to think about it yet. I want to exclude the cheap alternatives before I take the step and disassemble the engine block.

Quote:
Lean conditions cause back fire through the carburetor throat. Some folks refer to an after fire condition as back fire but it doesn't fit the definition very well. When the exhaust was mentioned, I could tell the meaning though.
No, the misfire does not come from the carburetor. That would make the noise louder, I think. It's hard to describe now, but chinacrackers in a tin can do it pretty well.

So actually I see the problem myself more in sucked false air (leaking manifold / carburetor seals) or actually in poorly closing valves.

I have to see if I can find someone with an endoscope who can look at the valves without taking my head off.
If there were clear deposits of oilcoal, the problem would have been found.



Greetings Andy

Last edited by DRG-ler; 07-27-2020 at 12:20 PM.
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Old 07-27-2020, 03:34 PM   #9
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Default Re: Backfire in overrun mode (load change)

Do you have a different known good carb you could swap in? Another cause of running lean is a worn throttle plate shaft/bushing. The quickest way to troubleshoot carb issues is trying another carb.



A little trick used by some is if the valves are sealing poorly, due to carbon, is to use a misting bottle with water and spray water into the carb air intake with the motor running.


If you suspect valves like you indicated using a borescope is a great idea, also might try performing a leak down/compression test.


If you suspect sticky valves try running Marvel Mystery oil in the gas. You may be able to see sticky valves by hand cranking the motor, look at each cylinder separately with a borescope. Otherwise removal of the valve access cover might be needed.


Just a few additional ideas that do not require head removal.
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Old 07-28-2020, 05:32 AM   #10
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Smile Re: Backfire in overrun mode (load change)

Add to all the above ... any change in the fuel mixture or a weak spark will make ign more difficult within the cylinder. On the overrun there is extreme suction sucking through a closed throttle plate and drawing in air from wherever it can, [air is more easily sucked through than fuel] making the mixture less than ideal. So add to the all above suggestions.... a weak or partially blocked idle mix, or an airleak through the threads of the idle jet ( I always smear a very small amount of hylomar blue on these threads).. Some back popple ( backfire, after-fire or whatever you want to call it ) on rapid down shifts or hard overrun is, I feel, inevitable with the primitive carburation of the Ford A; it is in the nature of the beast. I am sure I am not the only one who remembers the racing 7 litre Ford Galaxies (engineered and tuned to perfection) on the European (and no doubt USA) circuits in the 1960s giving an almighty backfire ( and flames as well) as they changed down and into overun from over 100 mph to less than 30 mph when approaching hairpin corners . The magnificent sound and visual image has lived with me for ever

Last edited by johnbuckley; 07-28-2020 at 03:06 PM.
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Old 07-28-2020, 09:22 AM   #11
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Default Re: Backfire in overrun mode (load change)

Thanks John ! I now understand what overrun is, I can hear it in my head
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Old 07-28-2020, 09:57 AM   #12
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Default Re: Backfire in overrun mode (load change)

Quote:
I am not familiar with "turbo brawl"
Methinks it may be a typo, maybe "turbo drawl"?
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Old 07-28-2020, 11:01 AM   #13
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Default Re: Backfire in overrun mode (load change)

Try re-torqueing the manifolds. What gasket did youu use on the manifolds?? Gland rings?? I liked the late 31 copper gaskets myself.
Paul in CT
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Old 07-28-2020, 01:04 PM   #14
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Default Re: Backfire in overrun mode (load change)

I used these manifold gaskets.

https://www.snydersantiqueauto.com/P...earchByKeyword

I've tightened manifold bolts more tightly than I wanted because the wrong torque was set, and I had noticed that too late. But were only 10Nm too much. In any case, all the same tight.

Quote:
Some back popple ( backfire, after-fire or whatever you want to call it ) on rapid down shifts or hard overrun is, I feel, inevitable with the primitive carburation of the Ford A; it is in the nature of the beast.
This is the best explanation ever !!! Everything is normal, can stay that way! Thumbs up!

I am primarily concerned with knowing whether the behavior is problematic and can cause damage. I mean, that this "problem" didn't exist when the car was bought.


To the carburettors ... a Zenith is installed, but I still have a Tillotson. But it's on the shelf, I don't know the condition yet. No idea if this is OK or needs to be overhauled. It is also questionable whether this is better. He will certainly be different.
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