Zenith - 1 Carburetor

jayvee34

jayvee34

Senior Member
Does anyone know if the Zenith - 1 carb is a tractor carb? It was on the car when I got it. Will only run if you use the tractor adjustment settings.

I rebuilt the carb and cleared all passages as per Tom Endy's 7 page article.
Results: Runs great if you use the tractor adjustments.
Here it is"
 

Attachments

No it is not a tractor carb. The picture is a tractor or forklift Zenith
adapted to a Model A.
 

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Bob C said:
No it is not a tractor carb.
Click to expand...

Can't imagine why it will only run on the tractor setting.
I rebuilt a tillotson for a backup, it starts at 1/4 open gav and runs at
50/55 MPH at 1/2 open gav. Guess I will run it. Thanks for your answer.
 
I have used a Zenith 1/carburetor on our 29 for almost 40 years. Runs great.

Enjoy.
 
follow Tom Endy's instructions, I found that he is the one to listen to about original model A zenith carbs if you wish to rebuild them ...
 
JD 1931 said:
follow Tom Endy's instructions, I found that he is the one to listen to about original model A zenith carbs if you wish to rebuild them ...
Click to expand...

I did, "The Zenith Carburetor"
(revised 2010) by Tom Endy
Step by step instructions, and illustrations. Still had to use the tractor settings
to get it to run. I rebuilt a Tillotson for a back up and put it on my car and it
starts right up a 1/4 turn CCW on the GAV, and runs on the road 50/55 MPH
with 1/2 turn on the GAV CCW. Over the past 65 years (87 years old) I have
rebuilt many carbs, so I believe that a previous owner has modified this carb
some way in order for it to behave as it does. It does run great at those tractor
settings, I just wanted it to run as a stock carb.
Thanks
 
Ok so I am really lost on this one. One person is talking about a Zenith 1 only running on tractor setting...Then someone is showing a picture of a Tillotson forklift carb. ??

What the heck are tractor settings? Does it have a GVA or not?
 
redmodelt said:
Ok so I am really lost on this one. One person is talking about a Zenith 1 only running on tractor setting...Then someone is showing a picture of a Tillotson forklift carb. ??

What the heck are tractor settings? Does it have a GVA or not?
Click to expand...

Yes it does. Go up to post #1 , I uploaded the Zenith Carb Adjustments which
is for a tractor. I don't know the tractor they are referring to. Check it out!
 
That link shows the instructions not the carb being talked about and did not see small type at bottom where it came from and line that I quote. Does your carb have the adjustable jet on the bottom (like the Tillotson)? "Set main jet adjusting needle at 4 1/2 full turns open." While not stock, it was an aftermarket item and could explain a lot. If so maybe change back to non adjustable jet and see if that helps.
 
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redmodelt said:
That link shows the instructions not the carb being talked about and did not see small type at bottom where it came from and line that I quote. Does your carb have the adjustable jet on the bottom (like the Tillotson)? "Set main jet adjusting needle at 4 1/2 full turns open." While not stock, it was an aftermarket item and could explain a lot. If so maybe change back to non adjustable jet and see if that helps.
Click to expand...

My Zenith -1 is suppose to be stock unless the previous owner modified it someway. Please excuse my ignorance of the adjustable jet. If you mean the GAV is adjustable, yes it is. I did use a rebuild kit for a stock zenith, and clean
soaked overnight in carb cleaner, and run a wire through all the passages.
After putting it back together and running it on my car it will only run as I described above, which is what the tractor adjustment describes. Any help concerning this is very much appreciated.
 
What jet are you turning 41/2 turns open? that is where I'm lost. I only know of the idle jet and the GAV.
 
Seems that everyone I have communicated with can't figure out this Zenith, I certainly cannot. I believe I will leave my backup Tillotson on the car which works perfectly and forget the Zenith for now.
Thanks to everyone that offered advice and help.
 
There are 3 adjustment screws normally on a stock Model A Zenith.

1. IAS Idle Air Screw on upper casting (45 degree angle) for adjusting Idle mixture.

2. GAV Gas Adjustment Valve lower casting (45 degree angle) on the same shaft as the choke lever.

3. The idle speed adjustment screw on throttle shaft


Then there is a 4th possibility.

4. A "T" handle on bottom coming out of the normal drain plug. (90 degrees to the casting)

This is not stock and if it is there it is the cause of this problem.

Which one are you turning 4 1/2 turns?
 
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A photo of this carb would tell us what it going on here.

Maybe it is not even a Model A carb although it does say Zenith 1.

Zenith made carbs for many different applications
 
Benson said:
There are 3 adjustment screws normally on a stock Model A Zenith.

1. IAS Idle Air Screw on upper casting (45 degree angle) for adjusting Idle mixture.

2. GAV Gas Adjustment Valve lower casting (45 degree angle) on the same shaft as the choke lever.

3. The idle speed adjustment screw on throttle shaft


Then there is a fourth possibility.

4. A "T" handle on bottom coming out of the normal drain plug. (90 degrees to the casting)

This is not stock and if it is there it is the cause of this problem.

Which one are you turning 4 1/2 turns?
Click to expand...


No T handle, and it is the GAV that I am adjusting. It is not 4 1/2 turns totally. Following the Zenith Adjustment that I uploaded in Post 1 I ended up
with the GAV 2 1/2 turns open. It idles and runs up to 50/55 MPH at 2 1/2 turns Open ,but will start and idle at 1/4 turn open. will backfire through the carb if exhilarated. Turn the GAV 1 to 1 1/2 turn it will run up to 35 MPH, Turn it another 1/2 turn it will exhilarate to 45 MPH. finally to 2 1/2 Turns open it will exhilarate easily to 50 MPH then slowly climb to 55 MPH. BTW No vacuum leaks.
 
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Sounds like a jet may be restricted. By opening the GAV that much I believe that's compensating for a blockage. If you want to mail me your jets I'll flow test them for you to see how far out of range they are. And if there's blockage I'll clear them to be the correct size.

We're taking our Model 'A' to Texas for the Texas tour 2021 in a few days so won't be able to get to it for a week or so. PM me if you want those flow tested and I'll send you my address.
 
John, Below from http://model-a.org/symptoms.html which is a good site for the Zenith carburetor. You are running too lean. If you cannot fix it they have a rebuilding service.



Mixture lean and over heating

Common Causes


  • Float is set too low.
  • Lower Casting Bowl Passage is restricted to Main Jet & Drain Plug.
  • Compensator Jet Orifice contains dirt or rust, or is too small.
  • Gasket at Main Jet is too thick or multiples are used, resulting in tip being too low.
  • Main Jet Orifice contains dirt or rust, or is too small.
  • Upper Casting Fuel Line Passage is restricted to Float Valve via Filter Strainer.
  • Lower Casting Comp. Jet Passage is restricted to Cap Jet via Secondary Well.
 
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