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Old 05-14-2018, 06:36 PM   #1
lorenallen
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Default Model A Zenith Carburetor

I have a 1931 Model A with a Zenith carburetor which leaks gas even with the manual valve shut off. I have changed many parts and I have cleaned the carburetor and had a good Model A mechanic look at it but it still leaks. It has to sit for a few hours to start leaking. I have changed all gaskets, The float valve (4 times), the float and shaft, the cap jet and main gets, and the main drain plug. Today I removed the carburetor and opened it up. I placed the lower halve in the vice and filled the gas in the bowl to about the level it would run at. after an hour gas was present on the top of the casting lip and after 3 hours the gas level dropped 1/2 inch in the bowl. The only place the gas accumulated was on the top edge of the casting and it eventually ran down the outside of the casting. I don't understand how this can happen - anyone have any ideas. Thank you for your response ahead of time.
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Old 05-14-2018, 09:39 PM   #2
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Default Re: Model A Zenith Carburetor

I think the first statement in your text answers your questions. The model A carb does not leak with the main gas valve shut off unless the main gas valve itself is leaking.
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Old 05-14-2018, 10:40 PM   #3
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Default Re: Model A Zenith Carburetor

Hairline casting fracture radiating up from the center through bolt bore. May not be visible at all. It is also possible that the casting itself is porous (a casting defect).


You could drive yourself nuts trying to find it or seal it but Zenith bottoms are plentiful. Replace it.
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Old 05-14-2018, 10:44 PM   #4
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Default Re: Model A Zenith Carburetor

A few places to look;

https://www.model-a.org/gas_leaks.html
https://www.fordbarn.com/forum/showpost.php?p=1136022&postcount=6
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Old 05-15-2018, 12:58 AM   #5
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Default Re: Model A Zenith Carburetor

A crack in the bowl would show gas weeping, read his post. Gas just sitting in the just the bowl which is resting in the jaws of a vice, the gas is creeping up the side and over the top.
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Old 05-15-2018, 06:50 AM   #6
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Default Re: Model A Zenith Carburetor

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Originally Posted by redmodelt View Post
A crack in the bowl would show gas weeping, read his post. Gas just sitting in the just the bowl which is resting in the jaws of a vice, the gas is creeping up the side and over the top.

I read the post. That is why I brought up the topic of fractures. Capillary action through a fine fracture will bring fuel up to the gasket face. So will porosity in the casting.
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Old 05-15-2018, 07:07 AM   #7
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Default Re: Model A Zenith Carburetor

My first step would be to replace the faulty leaking shut off valve - the "manual shut off".
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Old 05-15-2018, 08:59 AM   #8
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Default Re: Model A Zenith Carburetor

Is it recommended to replace the shut off valve or rebuild it? Mine is also leaking. Not sure if it's original or repro
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Old 05-15-2018, 09:09 AM   #9
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Default Re: Model A Zenith Carburetor

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Originally Posted by ryanheacox View Post
Is it recommended to replace the shut off valve or rebuild it? Mine is also leaking. Not sure if it's original or repro
generally why they leak is the brass is scored from microparticles of rust dirt rocks what have you. you can in theory lap them back together but even then gas and its capillary ways will find a path. Its relatively free to try to lap them back together but dont be surprised when it doesnt work well. When you install a new one or reinstall the old make sure you have a pencil filter in the top.
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Old 05-15-2018, 09:22 AM   #10
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Default Re: Model A Zenith Carburetor

ken & Ryan have it right also the shutoff valve in the carb may not shutting completely off but if the shutoff under the dash is good the carb would not be as important
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Old 05-15-2018, 10:12 AM   #11
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Default Re: Model A Zenith Carburetor

Another thing to consider is the float/fuel level. Adjust it with gaskets under the inlet valve

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Old 05-15-2018, 11:58 AM   #12
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Default Re: Model A Zenith Carburetor

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Originally Posted by SeaSlugs View Post
generally why they leak is the brass is scored from microparticles of rust dirt rocks what have you. you can in theory lap them back together but even then gas and its capillary ways will find a path. Its relatively free to try to lap them back together but dont be surprised when it doesnt work well. When you install a new one or reinstall the old make sure you have a pencil filter in the top.
Thanks for the advice. I can shut off the valve, run the carb dry and then restart it 10 minutes later and run it for maybe 30 seconds.

Sounds like I'll be ordering a new shut off valve.
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Old 05-15-2018, 12:06 PM   #13
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Default Re: Model A Zenith Carburetor

Be sure to have a pencil filter installed too.
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Old 05-15-2018, 07:12 PM   #14
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Default Re: Model A Zenith Carburetor

I pulled a Zenith apart and filled the bowl up to just below coming out the jet with mineral spirits. While the fluid did creep up the sides to the inside top edge, after 2 plus hours it had not gone over the top and down the sides. I would have tried gas but didn't have any handy.
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Old 05-16-2018, 06:01 AM   #15
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Default Re: Model A Zenith Carburetor

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Originally Posted by ryanheacox View Post
Is it recommended to replace the shut off valve or rebuild it? Mine is also leaking. Not sure if it's original or repro
I lapped my original leaking tank valve more than 20 years ago, and it still holds perfectly.
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Old 05-16-2018, 07:53 AM   #16
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Default Re: Model A Zenith Carburetor

Can't really argue with the price of lapping it so it'll be a good starting point at least
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Old 05-16-2018, 08:23 AM   #17
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Default Re: Model A Zenith Carburetor

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Originally Posted by Anteek29 View Post
Be sure to have a pencil filter installed too.
Not sure what's the point of those. If they ever clog, you still have to pull the valve.

Easy to check for leaky valve - just empty the sediment bowl & see if it fills up again.
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Old 05-16-2018, 08:38 AM   #18
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Default Re: Model A Zenith Carburetor

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Not sure what's the point of those. If they ever clog, you still have to pull the valve.

Easy to check for leaky valve - just empty the sediment bowl & see if it fills up again.
Purpose of filter in the tank valve is stop all the crud, mainly rust and sand from getting into shutoff valve.

For the pencil filter to get clogged the rust must be 1 1/2 or 2 inches deep in bottom of tank. Not very likely! By the time that happens you will notice that there is only 8 gallons in the tank ... the other 2 or 3 gallons will be taken up by the crud in bottom of tank.

Pencil filter also prevents larger pieces of crud from sliding along the bottom of tank where it covers up the opening in the tank shut off which stops flow of gas.

Then chunk of crud will slid away from outlet hole and the carburetor bowl will fill with gas again. Found a marble some kid dropped into tank which rolled around the bottom shutting off gas! THat will drive you crazy.

Finger filter stops marble from covering outlet!

I have been using these filters since 1969 in model A gas tanks ... Believe me they fix more problems than not.

Why cleaning sediment bowl does not address the PROBLEMs IF you do not have Finger filter.

By the time the rust goes through the shut off valve and gets to the sediment bowl, the shut off valve is full of rust and sand and dirt which soon damages the valve's brass parts.

Every time you move the valve more brass is ground off the valve stem and seat.

Usually it gets so bad that you can not turn the valve.

Pencil filter also prevents bits of cork from OLD gas gauge floats from plugging carburetor.

These cork pieces "sort of" float and get into float bowl and then inside the main and cap jets to plug them up intermittently.


Then under hard acceleration the flow of gas inside the cap Jet and main jet sucks the cork chunk up to top of the jet where hole reduces by 5O% or more this shuts off flow for that jet ... engine dies. Then cork chunk drops out bottom of jet, flows returns and engine runs again. Then flow increases. cork plugs jet again and engine dies again.

If either cap or main jet gets plugged at speed the engine dies immediately just like the key was turned off when the mixture goes to critical lean condition and dies.

The above is ACTUAL experience troubleshooting many carburetors and help from Bill Kenz and Paul Garrigan (Rumble Seat on V8 side.)

Last edited by Benson; 05-17-2018 at 07:38 PM. Reason: Many additions and corrections since this was posted first time
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Old 05-16-2018, 12:21 PM   #19
Purdy Swoft
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Default Re: Model A Zenith Carburetor

I consider the stand up pencil filter to be a must !!!
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Old 05-17-2018, 01:13 PM   #20
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Default Re: Model A Zenith Carburetor

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I consider the stand up pencil filter to be a must !!!
I agree with that.
I have repaired leaky shutoff valves using the kit vendors sell. They stop the leak and make the valve work as smooth as silk. Highly recommended. Cheap too!
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