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Old 10-03-2020, 05:53 PM   #1
AL in NY
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Default Modification to idle circuit in Zenith A and B carburetors

Has anyone tried adding a gasket between the idle jet and the upper carburetor casting to eliminate air leakage and increase the sensitivity of the idle air adjustment needle?
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Old 10-03-2020, 08:20 PM   #2
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Default Re: Modification to idle circuit in Zenith A and B carburetors

Instead of a gasket, I use a small bead of non-hardening gas-proof sealer on the jet where it contacts the cast iron body to ensure a good seal.

The idle jet has a tapered end that presses up against the cast iron threaded opening when the jet is screwed in tight. The brass tapered end gets chewed up by the edge of the casting when it seals. Over time it just doesn't seal very well.

I suppose a little gasket will work.

.
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Old 10-03-2020, 08:53 PM   #3
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Originally Posted by AL in NY View Post
Has anyone tried adding a gasket between the idle jet and the upper carburetor casting to eliminate air leakage and increase the sensitivity of the idle air adjustment needle?
Renners Corner is very familiar with the Model B Zenith, and others.
They may be able to help you also.
http://www.rennerscorner.com/carburetor.html
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Old 10-03-2020, 11:43 PM   #4
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Instead of a gasket, I use a small bead of non-hardening gas-proof sealer on the jet where it contacts the cast iron body to ensure a good seal.
Jim, what would that "non-hardening gas-proof sealer" be? Most sealers I have seen are gas resistant. It would be good to know what is gas proof. Thanks.
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Old 10-04-2020, 05:24 AM   #5
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Instead of a gasket, I use a small bead of non-hardening gas-proof sealer on the jet where it contacts the cast iron body to ensure a good seal.
x2 I use very small smear of Blue Hylpomar on the threads .
Intriguing I have come across one ( and only one) British manufactured Zentith carb wihich did not have a shoulder on either the casting or the jet and someone had used a gasket... I have not seen any other Zenith like that ( no matter where they were made) but I am still not sure if this was a backyard bodge or if it came out of the factory like that.
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Old 10-04-2020, 06:35 AM   #6
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Default Modification to idle circuit in Zenith A and B carburetors

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https://www.amazon.com/Valco-Cincinn...1811297&sr=8-3

“Allows metal-to-metal contact and is resistant to combustion products, water, engine coolant, petroleum, lubricating oils, kerosene and fluorocarbon refrigerants. Dresses gaskets, threads and seals gaps up to 0.25mm.”

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Old 10-04-2020, 08:43 AM   #7
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Default Re: Modification to idle circuit in Zenith A and B carburetors

I've rebuilt many Zenith carburetors and am always wary of "leaks" in and around all of the jets. I have to say, this area has never "to my knowledge" been a problem. Gaskets (on jets that call for them), length of jets, cracked jet tubes are always things to be concerned with. Air leakage around this "designed in" seal function on the idle jet ?????????? I would think that idle adjustments would be affected far more by a good set of flow tested and properly sized jet orifices and that this would far overtake a need for concern over whether the idle jet mating tapers would be cause for concern. Then, understanding and tweaking the sizing of corresponding jets and "playing with" those sizes would be a far better practice to super tuning the carburetor. I understand many of us want that idle so low that we can "count off" the cylinders as they fire. Taken into overall considerations of how to make that happen, air leakage at the idle jet mount would seem at the bottom of those considerations. And, don't forget, overall condition of the engine will also affect how well any carburetor will adjust and function.

This is just my experience talking. "Individual results may vary".

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Old 10-04-2020, 09:31 AM   #8
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Jim, what would that "non-hardening gas-proof sealer" be? Most sealers I have seen are gas resistant. It would be good to know what is gas proof. Thanks.
I use Hylomar AF. The original Hylomar Blue would work.

This jet sealing surface is not submerged in gasoline all the time.

Bob, the sealing of a new jet is good. I like to use original jets whenever I can and the tapered ends are often chewed up and don't seal well. It may be overkill.

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Old 10-04-2020, 11:57 AM   #9
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Default Re: Modification to idle circuit in Zenith A and B carburetors

Quote:
Originally Posted by shew01 View Post
https://www.amazon.com/Valco-Cincinn...1811297&sr=8-3

“Allows metal-to-metal contact and is resistant to combustion products, water, engine coolant, petroleum, lubricating oils, kerosene and fluorocarbon refrigerants. Dresses gaskets, threads and seals gaps up to 0.25mm.”

Sent from my iPhone using Tapatalk
Quote:
Originally Posted by Jim/GA View Post
I use Hylomar AF. The original Hylomar Blue would work.

This jet sealing surface is not submerged in gasoline all the time.
Thanks, but I see it resists gasoline but is not gasoline proof.

The reason I asked is I have a Marvel Shebler carby that insists on weeping out the gasket between the upper and lower sections. Thought I might try some sort of sealer, but I don't think a gas resistant sealer would do the job. The gasket surfaces are flat now after I filed them but were warped before.

Thanks.
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Old 10-04-2020, 12:12 PM   #10
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Thanks, but I see it resists gasoline but is not gasoline proof.

The reason I asked is I have a Marvel Shebler carby that insists on weeping out the gasket between the upper and lower sections. Thought I might try some sort of sealer, but I don't think a gas resistant sealer would do the job. The gasket surfaces are flat now after I filed them but were warped before.

Thanks.
I don't know gas proof vs. gas resistant. I know it's hard to clean it off of my tools when it gets on them. I have to use spray brake cleaner solvent on a rag.

To quote a TV commercial for hot sauce "I use that s#!t on everything!".

I have not had anything that I used it on leak after applying a thin coating of Hylomar AF. Many of those applications are fully submerged in gasoline, oil, antifreeze, etc.

It is great inside the radiator hoses to help slip them on and to help keep them from weeping (without having to go crazy tight on the hose clamp). They also come off easier later, because it never hardens.

I have not used it on the carb body gasket, but I am sure that if I did, that carb would not weep. I use it on all the other gaskets on the carbs. I hate gas dripping off of the drain plug of a carb (the lowest point). Flattening the warped gasket surfaces true again is certainly important.

.
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Old 10-04-2020, 12:59 PM   #11
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Default Re: Modification to idle circuit in Zenith A and B carburetors

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Originally Posted by Jim/GA View Post
I don't know gas proof vs. gas resistant. I know it's hard to clean it off of my tools when it gets on them. I have to use spray brake cleaner solvent on a rag.

To quote a TV commercial for hot sauce "I use the s#!t on everything!".

I have not had anything that I used it on leak after applying a thin coating of Hylomar AF. Many of those applications are fully submerged in gasoline, oil, antifreeze, etc.

It is great inside the radiator hoses to help slip them on and to help keep them from weeping (without having to go crazy tight on the hose clamp). They also come off easier later, because it never hardens.

I have not used it on the carb body gasket, but I am sure that if I did, that carb would not weep. I use it on all the other gaskets on the carbs. I hate gas dripping off of the drain plug of a carb (the lowest point). Flattening the warped gasket surfaces true again is certainly important.

.
Thanks Jim for your reply. I certainly don't have anything to lose if it doesn't work, besides clean-up time. So I think I will give it a try. Thanks again.
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Old 10-05-2020, 09:44 AM   #12
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Default Re: Modification to idle circuit in Zenith A and B carburetors

Folks, I am talking about the UPPER carburetor casting fit to the wide shoulder of the idle jet, not the threaded tapered fitting in the lower casting. There is a chamber in the upper casting where fuel is mixed with air before being drawn into the carburetor throat. This chamber should only receive air through the idle air adjustment needle valve. The fit of the idle jet into the upper casting is a loose sliding fit which would allow air to be drawn into the mixing chamber, bypassing the idle air adjustment needle valve. I wanted to know if anyone had tried adding a gasket to the wide shoulder of the idle jet to prevent this air leak. Preventing this air leak would increase the sensitivity of air adjustments made at the needle valve. This question/modification is directed at the B carburetor, I was assuming the A carburetor was similarly built.
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Old 10-09-2020, 12:41 PM   #13
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Default Re: Modification to idle circuit in Zenith A and B carburetors

Folks, I added a gasket between the idle jet shoulder and the bottom of the upper casting. I does improve the idle mixture sensitivity to the adjustment of the air adjusting screw. The one problem I found is that the space between the idle jet shoulder and the bottom surface of the upper casting varies between .080" and .060". You'll have to use a very small piece of modeling clay to determine your measurement. The Model A/B carburetor gaskets a very hard and don't compress very much, so your measurement has to do fairly accurate. Use gaskets that measure about .005" more then your measured spacing. I think the best solution would be a small .090" thick viton O-ring, because the O-ring will compress to the required thickness.
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