Model A Zenith Carburetor

[lorenallen]

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.

[bucket-o-rust]

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.

[MikeK]

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.

[Mike V. Florida]

A few places to look;

https://www.model-a.org/gas_leaks.html
https://www.fordbarn.com/forum/showpost.php?p=1136022&postcount=6

[redmodelt]

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.

[MikeK]

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Originally Posted by redmodelt

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.

[kenparker]

My first step would be to replace the faulty leaking shut off valve - the "manual shut off".

[ryanheacox]

Is it recommended to replace the shut off valve or rebuild it? Mine is also leaking. Not sure if it's original or repro

[SeaSlugs]

Originally Posted by ryanheacox

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.

[jm29henry]

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

[1930artdeco]

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

Mike

[ryanheacox]

Originally Posted by SeaSlugs

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.

[Anteek29]

Be sure to have a pencil filter installed too.

[redmodelt]

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.

[Tom Wesenberg]

Originally Posted by ryanheacox

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.

[ryanheacox]

Can't really argue with the price of lapping it so it'll be a good starting point at least

[Badpuppy]

Originally Posted by Anteek29

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.

[Benson]

Originally Posted by Badpuppy

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.)

[Purdy Swoft]

I consider the stand up pencil filter to be a must !!!

[Synchro909]

Originally Posted by Purdy Swoft

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!

[Fred K-OR]

Originally Posted by Badpuppy

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.

They stick up about an inch or so in the tank. Would take a lot of junk in the tank to block this filter.

[Fred K-OR]

Originally Posted by Tom Wesenberg

I lapped my original leaking tank valve more than 20 years ago, and it still holds perfectly.

Tom, please tell this guy that does not know much about this subject, what you mean by "lapping". Sorry, I think you may have done this before but the old mind forgot.

[Tom Wesenberg]

I don't remember if I spun it some in my lathe and used a little crocus cloth to help polish it, but I do remember using toothpaste and turning it by hand. I think I may have used fine valve grinding compound, then toothpaste. I then gave it a thin coat of grease and installed a new shaft seal kit from one of the parts houses, Bert's, Bratton's, and Snyder's.


Before doing this the handle was hard to turn, and ever since the lapping and greasing, it is very easy to turn.

[Growley bear]

Originally Posted by MikeK

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.

I have repaired a few of the newer manufactured Zenith carburetors and noticed that the inside of the housing has a dull black coating. Is that a process by the manufacturer to help eliminate housing porosity? I know that through capillary action, gasoline, with it's very low viscosity can/will find the smallest pinhole in a casting.

[Growley bear]

Originally Posted by SeaSlugs

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.

Been there, try as I did, the time spent lapping and trying to seat the old valve was an education in futility. Hours were spent and in the end a new valve was installed and problem solved. I've been doing this sort of repair work for over half a century but I had to try. For the price of a valve, if the old one needs more than cleaning I would replace it. The voice of experience says that lapping two equally scored surfaces together will not end up with a smooth leak free surface. Not only will they leak to the carburetor but the stem can also leak inside the passenger compartment.

[redmodelt]

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Originally Posted by redmodelt

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.

Follow up on test; After almost 3 days, the mineral spirits are pretty much at the same level in the bowl except for some evaporation and only a couple of damp spots on the face where it is machined for the top.

[Fred K-OR]

Here is a quick fix for a leaky gas shut off value. I put one of these things on my huckster years ago. The short term advantage is that you do not have to drain your gas tank. But the long term disadvantage is that you have to turn on two dxx values every time you start your rig. Most vendors carry these things if you are interested. This is on my 29. Don't know if these work on 30/31's.

[Growley bear]

I use mineral spirits when bench checking float levels and float valves. Works well and I think safer than gasoline albeit still flammable. It has always worked well for me.

[Ernie Vitucci]

I have the shut off valve that is shown in post 27 and since I installed it, I have not moved the original shut off valve at all. I just leave it open and use the new one which is a 1/4 turn ball valve and much better than the original. It will not leak and so when I park our A at night, in the garage, I shut off the gas and let the engine run for a couple of minutes and then shut it off and have no gas smell or leak at all. If you happen to have a carburetor that leaks through the casting and you don't want to purchase another one right now, then when you turn off the gas with the new valve, just run the engine until it empties the carburetor completely and stops running. When you next drive your A, turn on the new gas valve and let gas run down into the carburetor before you crank it over. Ernie

[redmodelt]

So far only one person has posted a link that talks about float level. If the float level is correct and the float valve at the carb is working it will or should not matter if the shut off valve is left open or closed. Unless the float valve is not sealing to the upper casting or there is a crack or porous casting. No word from original poster.



Lots of good information given here, but it has been 6 day and not a word from the original poster. I for one am wondering if the question is for real. I see a number of postings in the "Questions and Suggestions" section by one time posters that A; on the surface look real but generally are just a little off and B; when told to ask in the correct forum never do. I hope this is not the case and any and all information that has been give help him or others.

[lorenallen]

I have read all the posts and as a result I went back to check the manual shutoff valve. I removed the gas line between the settling bowl and carburetor and hung a can to catch any leaks. The first overnight I had about a tablespoon of gas in the can and the second night I had about 3/4 of a teaspoon in the can. I decided to order an electric shut off valve from Snyders to try this. I will install it when I receive it let you know. The present manual shutoff valve is less that 1 year old.

[Tom Wesenberg]

Are you using the screen in the tank valve?
I consider it a "must have".

[lorenallen]

Yes, I have the screen in the tank valve, the screen in the carburetor and a cartridge filter in the line after the settling bowl. I want to thank everyone for their input on this problem and I have learned a few new things to check from your input.

[lorenallen]

I am new to this forum so I am sorry that I did not reply sooner. I guess it did not occur to me to reply sooner and I will try to respond sooner in the future.

[redmodelt]

No problem, glad to see that you were still out there.

[Growley bear]

Originally Posted by redmodelt

So far only one person has posted a link that talks about float level. If the float level is correct and the float valve at the carb is working it will or should not matter if the shut off valve is left open or closed. Unless the float valve is not sealing to the upper casting or there is a crack or porous casting. No word from original poster.



Lots of good information given here, but it has been 6 day and not a word from the original poster. I for one am wondering if the question is for real. I see a number of postings in the "Questions and Suggestions" section by one time posters that A; on the surface look real but generally are just a little off and B; when told to ask in the correct forum never do. I hope this is not the case and any and all information that has been give help him or others.

I believe that the fuel shut off valve is/was an added precaution due to the location of the fuel tank. This was a common practice on most machinery that had a gravity feed fuel system. My friend who is 91 has owned his Model A for 5 years and has never closed the valve and his carburetor has never leaked a drop. The B2 carburetor on my Model A doesn't leak either, but I still shut it off. My problem is remembering sometimes to turn it on. Kind of embarrassing in a busy parking lot.