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Old 06-07-2022, 02:40 PM   #1
DRG-ler
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Default Backfire, I'm running out of ideas...

Hi guys, my A is giving me some problems.

It's consuming too much and backfiring on coasting/decelerating.
What has been checked so far... A pressure drop test was done which showed no problems. Inlet valves, outlet valves, piston rings were tight.
The engine got a new head gasket. A visual inspection of the valve seats was also carried out there. Also without any abnormalities. Also, no stuck/hanging valves were found when turning by hand. (The problems existed before the head gasket was replaced.)


------------------------------------------------

ignition

Ignition was set according to the setting instructions and checked with a flash gun. Ignition timing with the lever at the top is 0 degrees.
Capacitor on ignition distributor was measured and declared to be functional.


------------------------------------------------

Carburetor

Carburettor got new jets. Jets are flow tested by David Renner.
With the new jets, the engine runs much cleaner and quieter, and starts better. But the misfires remain!!!
Only the running was tested when stationary, not while driving! Furthermore, the float level was readjusted. The Schmimmer is now at 15.5 mm with the needle valve fully closed. About 5/8" as I could see in a YT video. Measured to the carburetor body separation. Furthermore, it makes no difference whether I screw the idle mixture screw all the way in or out. The engine doesn't run any differently, and it doesn't stop either!! Even a change to the GAV does not bring any change! (Should actually only be under load / driving.) Loud popping / misfires when going down EVERY gas shock!


----------------------------------------------

manifold

The intake/exhaust manifold was reground together at the engine builder. This one is straight again. The seals are new and seated correctly. The manifold was sprayed with brake cleaner while the engine was running. He draws no secondary air, is not leaking! The carburetor flange was also sprayed off and is also tight.
The only thing that hasn't been replaced yet is the breaker contact. However, that's what my previous owner did. I think it hasn't done 5000km yet. The modern version has already been retrofitted.
Now the question is, where are the misfires coming from? These do not occur when driving under load when accelerating. Only when decelerating.


Greetings Andy
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Old 06-07-2022, 02:48 PM   #2
Eccentric Old Guy
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Default Re: Backfire, I'm running out of ideas...

Checking in to follow this thread....

eog
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Old 06-07-2022, 02:48 PM   #3
daveymc29
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Default Re: Backfire, I'm running out of ideas...

When you know it's gas related, try to fix the electrical problem. How about a condenser for a starting point?
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Old 06-07-2022, 02:53 PM   #4
DRG-ler
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Default Re: Backfire, I'm running out of ideas...

I already wrote above. The capacitor isn't old yet, but it was measured anyway and it's OK. It's not because of these. Spark plugs are also new. Only the breaker contact would have to be renewed, but this is apparently not very old either.
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Old 06-07-2022, 03:05 PM   #5
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Default Re: Backfire, I'm running out of ideas...

An ignition problem could cause miss and/or after fire. A cracked distributor head or carbon tracing there is a possibility. A sloppy distributor bushing can cause a miss. A bad wire between the condenser plate and the rotating breaker plate can cause intermittent ignition. An intermittent ignition on/off switch can cause problems.

A leaky exhaust manifold can be problematic. You wouldn't think it could but a mixture can ignite in there better if fresh air can get in on shut down of the engine.
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Old 06-07-2022, 03:12 PM   #6
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Default Re: Backfire, I'm running out of ideas...

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Deleted. He already addressed.
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Old 06-07-2022, 03:16 PM   #7
DRG-ler
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Default Re: Backfire, I'm running out of ideas...

One more note from me. Before replacing the carburetor jets, the engine was running far too rich. The problem was sooty spark plugs and valves/cylinder heads. In the meantime I did a 200km test drive with a friend's loaner carburetor. This burned away all the black charcoal residue and fuel consumption dropped noticeably. After that I did a 160km test drive with my carburettor with the old jets and closed the GAV completely! Coal did not form, but consumption was extremely high again! That's why I got the flow-tested jets from David Renner. I hope that this will reduce consumption. Currently with the old nozzles I have only managed 160km with more than half the tank filling.


I also forgot to mention that the distributor was refurbished. The housing broke off when replacing the cylinder head gasket. Therefore, the distributor housing was exchanged. It is an old used housing, but the shaft runs in the bearings without play. A hole for lubricating the upper bearing in the shaft and screw has been retrofitted. The shaft in the engine block is new. The connector of the distributor shaft was renewed. The cable between the lower and upper plate is perfect.


In retrospect, it occurs to me that a first attempt to start the engine after replacing the head gasket was unsuccessful. The connection of the armored cable in the distributor had a short to the housing somewhere there. However, after removing the distributor plates, I could not locate it. When you put it back together the short was gone. Maybe I should look again at this point...
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Old 06-07-2022, 03:32 PM   #8
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Default Re: Backfire, I'm running out of ideas...

Quote:
A leaky exhaust manifold can be problematic...
So definitely the seal/connection intake/exhaust manifold is tight. This was sprayed with brake cleaner. Between the manifold and the exhaust he could get a minimal amount of secondary air. Since I haven't installed a centering sleeve here yet (it's already here.), it's possible that the seal has slipped a bit. I have to see, I built an adapter to install a lambda probe, which is screwed between the exhaust manifold and the exhaust. however, I am still missing the blind plug for driving without a probe, since this is only to be used for setting. When I have the plug, I'll block everything and additionally center/seal.

On the other hand, how can a leaking exhaust cause a misfire? That would mean that the exhaust valves would not close correctly, and the leaking exhaust would suck in false air through the closed exhaust valves. Then the valve springs would have to be very weak!
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Old 06-07-2022, 04:04 PM   #9
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Default Re: Backfire, I'm running out of ideas...

What I picked up from all of this is that your idle adjustment is not making any changes. The GAV only functions when the throttle is open. Is it still true that the idle adjustment is not making any difference?

When you are going down a hill or when coming to a stop, or just idling at the car, the idle circuit is in play. If that part of the carburetor is not functioning correctly it can cause a backfire. Essentially raw gasoline is getting into the muffler and igniting by hot exhaust or a hot spot in the muffler.

Make sure the throttle is shutting completely. Screw out the idle speed adjustment screw to see if the engine will stop. If not, adjust the throttle linkage by bending the rod or some other adjustment. Sometimes the throttle mechanism can be hanging up on the floor boards. Check the throttle shaft for looseness.

Borrow your friends carburetor again and see if the problem goes away. If not, then it is likely in the ignition system. Look for carbon tracks or other problems that Rotor Wrench talked about (post #5).

https://model-a.org/default.html may help.
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Old 06-07-2022, 04:12 PM   #10
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Default Re: Backfire, I'm running out of ideas...

I had a car that would afterfire when shut off. It was leaky pipe to manifold fitting.

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Old 06-07-2022, 06:14 PM   #11
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Default Re: Backfire, I'm running out of ideas...

I had a problem with the lead from my alternator. The connecting nut was tight but the wire within the clip was not. I replaced the alternator with a generator and the problem remained, and the car would run fine when first started for the day. I would drive the first tank of gas out of car for the day then refill. It was a "Belch Fire Special" for several miles until eventually I could nurse it along and get it up to roadspeed. Then it seemed okay for most of the rest of the second tank of gas, we were going to TX and not using a third fill up most travel days. I had less trouble on the way home. but it was still not right. Two years later we decided to find out what the problem had been and a friend touched the war from my lternator, which I had taken to be renewed and repaced on my car. He said he saw the wire move, we pulled it out with no effort so I put it back, after I cleaned both the clip and the wire strands, sqeezed it tight and it had worked fine ever since, I did solder it to insure it didn't fail again. No more Belch Fire.
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Old 06-07-2022, 06:52 PM   #12
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Default Re: Backfire, I'm running out of ideas...

I agree with Nkaminar.
Switch out the carburetor. If it gets better then the problem is there. Check to make sure that one of the old gaskets on the main jet or cap jet didn't get inadvertently left in the carb changing the height of the jet.

If switching the carb doesn't help then switch out the distributor with a known good running one.
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Old 06-08-2022, 01:04 AM   #13
DRG-ler
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Default Re: Backfire, I'm running out of ideas...

Quote:
Make sure the throttle is shutting completely. Screw out the idle speed adjustment screw to see if the engine will stop. If not, adjust the throttle linkage by bending the rod or some other adjustment. Sometimes the throttle mechanism can be hanging up on the floor boards. Check the throttle shaft for looseness.
I'll go after that again today. I removed the throttle body to clean it. It also showed that it is easy to move. However, it is open a small gap for the idle air. Does the flap to adjust the idle mixture screw have to be 100% closed? Then I'll check again today.


@daveymc29, I already have a generator installed. I can rule out the problem.
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Old 06-08-2022, 05:30 AM   #14
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Default Re: Backfire, I'm running out of ideas...

After you screw out the idle speed adjustment screw to test whether the throttle shuts off completely, you will have to screw it in so that the engine idles. The instructions from Ford were to retard the spark and then adjust the idle speed and mixture for a good idle then advance the spark to the normal position. The throttle plate has to be open a small gap for the car to idle. The purpose of all of this is to see if the accelerator linkage is preventing the carburetor to go to the idle position. If it does not go to the idle position then you cannot adjust the idle mixture and this may be causing the backfire. If there is something wrong with the idle circuit in the carburetor, for instance a wrong or clogged jet somewhere, then this could be causing the backfire. The idle circuit is more complicated than just getting fuel to the small hole in the side of the throttle body. There is part of it that mixes air into the fuel before getting to the little hole. See https://model-a.org/default.html
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A is for apple, green as the sky.
Step on the gas, for tomorrow I die.
Forget the brakes, they really don't work.
The clutch always sticks, and starts with a jerk.
My car grows red hair, and flies through the air.
Driving's a blast, a blast from the past.
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Old 06-08-2022, 06:08 AM   #15
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Default Re: Backfire, I'm running out of ideas...

Separate your carburetor top and base. Look for a hairline Crack along the gasket surface of the base.
I have found a few zenith carburetors with a crack. When hot the crack expands,this will give you a fit.
One A, I could not get it to run right,manifolds were shaved and all was perfect.
Well I swapped both out, problem solved! So I the bought a New exhaust manifold and installed it and all was well.
I do not shave the manifolds at all, or torque them down. ,I feel this is wrong advice. Might be just me.
But I have cracked a few by doing using this technique. Swap your manifolds.
Last is the timing....if you use the original timing pin,make sure it's Not bent! I found 3 that were bent recently. People bend them when prying against the engine when installing or removing the front mount.
Last, check your pop out cable at night with the engine running,wiggle it. Look for anything arcing.
Thsee are all the little thing I've found over the years.
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Old 06-09-2022, 08:34 PM   #16
Sunny the Model A
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Default Re: Backfire, I'm running out of ideas...

My model a will backfire a bit on deceleration, just a little puff sound because of a slight gap in the manifold joint. i does have a sleeve installed to help and it does, but I won't put sealant on it because I take the heater manifolds off during the summer and put it back on in the winter. if it is slight, run with it, if it sounds like war is going on, look into it.
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Old 06-09-2022, 10:14 PM   #17
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Default Re: Backfire, I'm running out of ideas...

I wonder if this is an ignition problem, because I had a similar issue a couple of years ago. The problem turned out to be a short of the ignition wire inside the armored cable. The original wire was still in the armored cable and the insulation had deteriorated badly.

I had another similar issue that turned out to be loose electrical connections in the terminal box on the firewall and on the ammeter connections. If those connections are even slightly loose, they will heat up as the engine runs and become intermittent.

Good luck figuring this out.
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Old 06-10-2022, 12:44 AM   #18
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Default Re: Backfire, I'm running out of ideas...

Quote:
I wonder if this is an ignition problem, because I had a similar issue a couple of years ago. The problem turned out to be a short of the ignition wire inside the armored cable. The original wire was still in the armored cable and the insulation had deteriorated badly.


That's a mistake I've been thinking about too. My armored cable is in similar bad condition. But my question is whether the problem shouldn't also be misfiring while driving / accelerating?! That's definitely not!?
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Old 06-10-2022, 02:25 AM   #19
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Default Re: Backfire, I'm running out of ideas...

Well, no one's mentioned it so far so I will! .... try a different brand of fuel
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Old 06-10-2022, 02:40 AM   #20
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Default Re: Backfire, I'm running out of ideas...

If you haven't done already check if your carb is one of the early ones with the brass seat in the GAV-circuit. It's hard to see, you can also check if there's a thread in the seat area. (I think M5, but I'm not sure anymore)
When your missing the bass GAV-seat, the GAV is always in fully open position and your fuel consumption will be too high. Additionally turning the GAV obv. won't make a difference and the plugs will foul quickly.


Don't ask me how I know all that.
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