Backfire, I'm running out of ideas...

[CT Jack]

In my opinion you have 2 main component issues to resolve, carburation and ignition.
Carburation:
With the engine idling you should be able to change the smoothness of the idle when turning the idle screw. The full adjusting range for this screw should go from stalling the engine to running rough (rauh). I think you have a problem with the GAV. If it is working properly you should be able to hear a slight change in the idle speed as it is turned in and out. If this jet or valve is not working properly you could be getting a large amount of fuel flow going directly to the high speed chamber. This also could be why you do not get any response from adjusting the idle screw. It would definitely affect fuel consumption. My GAV setting is 1/4 turn open. I recommend you install another Zenith carburetor that you know works for a comparison test. You should see an immediate change in results.
Electrical:
Do not screw the armor cable too far into the distributor housing. It only needs a couple of turns. Going too far in can force the plate contact to touch the distributor housing causing it to ground out and prevent starting the engine. It could also cause an occasional backfire if it is very close to the housing. I would change the condenser. It could be failing when it gets hot. I would also make certain the timing is correct.
Viel Gluck

[DRG-ler]

So... I tried to adjust the carburetor. I turned the idling screw almost out so that the throttle valve was almost closed. Engine ran very, very slowly. Nevertheless, adjusting the idle mixture screw brought very little. But it was already having a small effect. Nevertheless, it popped out of the exhaust when the engine went down! Also, very large clouds of black soot came out of the exhaust. When I opened the GAV a full turn, the misfires were almost gone. At two revs, the backfire misfires were completely gone.

No more popping, no clouds of soot.

I have a feeling that the float level might be too low. I had lowered the needle valve with a second seal so that it closes earlier. I'll change that again and attach a control hose to the carburettor to check the float level.

[Chuck Sea/Tac]

Or your main jet is too small

[daveymc29]

Originally Posted by DRG-ler

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.

I did also, replaced the alternator with a generator, then went back to the alternator. All the time it was the clip on the wire not the appliance supplying the 6V.
I loaned the generator to another guy going to Addison and he returned it when we got there and he found a vender. Still have it and also still running the 6V Alternator.

[bavArian]

You did make sure that all passageways are completely free and not blocked by god-knows-what, right? Sounds a bit like one of the passageways is constricted. My car runs right between 1/4 and 1/2 turn GAV. You shouldn't need a lot more when the engine is warm.

[DRG-ler]

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Soooo, new day, new try.

Today I completely disassembled the carburetor. It is currently in the ultrasonic bath for cleaning. Will take a while.

In parallel, I took my Tillotson carburetor out of storage and assembled it to test it. I had to realize that it must be a Model B carburetor, because the mounting flange is at an angle. This means that the fuel level in the carburetor is not correct. It seems to be running way too rich because I have to leave the choke 80% open to keep the engine from stalling. Since I do not have a suitable intake manifold, the carburetor is not usable for me. What surprises me is the connection of the fuel line, which is connected like with the Model A. Normally the Model B had a fuel pump and the connection from the front. Will probably jmd. have thrown different carburetors together. In any case, I can't get any further with these carburetors.

Can someone tell me off the top of their head which thread the screw at the bottom of the Zenith carburettor has that closes the jet? I want to see if I can build a hose connection there to check the fuel level in the carburetor. The finished sets that you get thrown at every corner in the USA are far too expensive here in Germany.

[Ed in Maine]

Start the car at night and look for any arcing around the distributer. Change the distributer body and cap. I know you mentioned that you have flow tested jets but recheck the Main Jet to verify a flow rate of 150-160 ml/min and the Idle Jet is at 44-48 ml/min. Ed

[History]

I took the bottom half off of my zenith and with it installed on the car zip tied a small glass jar to the bottom of the zenith and could see the exact level of petrol. I had someone turn the gas on while I watched so I could instruct them to also turn it off if it overflowed. It worked well for me because I had the pieces to do it that way and did not have the parts to do it as others have. Obviously you need to be careful playing with an open source of petrol. I would see if you could borrow your friend’s carburetor again and narrow it down before I did much more or waste money. Good luck and keep us informed.

Originally Posted by DRG-ler

Soooo, new day, new try.

Today I completely disassembled the carburetor. It is currently in the ultrasonic bath for cleaning. Will take a while.

In parallel, I took my Tillotson carburetor out of storage and assembled it to test it. I had to realize that it must be a Model B carburetor, because the mounting flange is at an angle. This means that the fuel level in the carburetor is not correct. It seems to be running way too rich because I have to leave the choke 80% open to keep the engine from stalling. Since I do not have a suitable intake manifold, the carburetor is not usable for me. What surprises me is the connection of the fuel line, which is connected like with the Model A. Normally the Model B had a fuel pump and the connection from the front. Will probably jmd. have thrown different carburetors together. In any case, I can't get any further with these carburetors.

Can someone tell me off the top of their head which thread the screw at the bottom of the Zenith carburettor has that closes the jet? I want to see if I can build a hose connection there to check the fuel level in the carburetor. The finished sets that you get thrown at every corner in the USA are far too expensive here in Germany.

[katy]

Originally Posted by DRG-ler

Can someone tell me off the top of their head which thread the screw at the bottom of the Zenith carburettor has that closes the jet?

The one to the rear, It's directly centered under the throttle plate.
Picture courtesy of?

[johnbuckley]

[QUOTE I took the bottom half off of my zenith and with it installed on the car zip tied a small glass jar to the bottom of the zenith and could see the exact level of petrol. /QUOTE]
Yes I too prefer a glass jar to a tube gauge ( but not so easy on RHD cars)... with a jar you can see what's happening . A 3 inch high , 3 inch diameter jar is ideal : go into your local supermarket with a tape measure and check out the shelves ( you'll get weird looks!) over here the ideal size jar contains Greek olives. Eat the olives then play with the car.

[kimlinh]

Hi DRG
I had a problem similar to what you are describing. My car started out as a complete basket case. Every part was rusty. I cleaned and derusted everything including the carburetor. The problem I had was caused by the enlargement of the idle passageway and the idle port in the throat of the carburetor. It was the cast iron body of the carburetor itself that caused my problem. I replaced the carburetor body and now my car runs great. I hope this helps.

Steve

[nkaminar]

I have been reading Ricardo's book written in 1931. Here is what he says about backfiring, summarized in my words.

When the fuel mixture is leaned out, for instance by a partially plugged jet, the air/fuel mixture burns more slowly. The mixture can still be burning when the exhaust valve and intake valve are both open which leads to ignition of the air/fuel in the intake manifold. It also leads to incomplete combustion which can lead to raw air/fuel in the exhaust manifold and ignition of that in the muffler/exhaust system.

He also wrote that a slight lean mixture is the most economical but that for power a slightly rich mixture is best. So open up the GAV when climbing hills and close it down when cruising down the highway. The rich mixture will cool the incoming air and cause the engine to run cooler.

Update: Ricardo also wrote that a very rich mixture can slow down the burning too and cause backfiring in the same way that a very lean mixture can cause backfiring.

[DRG-ler]

Soooo, another new day, screwed on again.

After cleaning the carburettor completely in the ultrasonic bath yesterday, I reinstalled it today. With my neighbors, also vintage car drivers, we built a float-level display to check this. When assembling the carburettor, I immediately removed my additional seal under the needle valve, and when I checked the float level was exactly right!

After the misfiring could be heard again during the test run, we tested each cylinder individually with a glass spark plug to see from the color of the combustion whether the mixture was too rich or too lean. This visual check is certainly not ideal, but at least gives initial tendencies. In any case, the mixture formation did not look bad. There's no such thing as too lean or too rich. Then I opened the GAV 2 turns while the engine was running and enriched the mixture extremely. There you could see a very yellow burn in the glass spark plug. However, you could see this not only on the glass spark plug, but also on the exhaust manifold gasket on cylinder 4!!!
That really annoys me!!! You could clearly see the flame at the seal. I was also annoyed that you didn't notice or at least hear about the spraying with brake cleaner. The problem is that the stuffing box rings don't fit in the manifold because it doesn't have the fitting for them. This is present in the engine block. In the foreseeable future I will probably go back to the engine builder with the manifold and have the mount for the rings re-milled in order to finally get this miserable manifold and exhaust pipe sealed!!!

[Rob Doe]

If I understand you correctly, you have identified a major exhaust leak???

The metal gland rings are not necessary. Is your exhaust manifold an older one? If so, the alignment of the intake and exhaust port openings are often misaligned i.e. the openings are drooped, especially at exhaust port #4.

You planed the surface, but the drooping cannot be repaired. Repair attempts end up in cracking the manifold. Delete the gland rings and reinstall the manifold. Retest for leaks afterward.

[nkaminar]

Lay a straight edge on the exhaust manifold when it is out of the car to check for flatness and drooping at the #4 exhaust port. If you use the rings they must be checked for fit by test mounting the manifold without any gasket. There should be no gap. You can shorten the rings if needed. Check the flatness of the exhaust manifold and intake manifold when bolted together.

I have had the best luck with the original style gaskets, see link below, and using a sealant, see link below. Tighten the nuts on the studs to 55 ft-pounds and check them again after running the engine. Check a couple of times.

The leak in the exhaust system can lead to backfiring.

https://www.snydersantiqueauto.com/p...4052&cat=41733
https://www.snydersantiqueauto.com/p...8537&cat=41736