Zenith Carburetor Question - Venturi and more
 

Carburetor Question: Can you use a Single Venturi with an Inside Diameter of 0.675” (Typical is .845” ID) on a carburetor designed for Double Venturi, or should the .875” ID be used?

I am working on a 1930 Tudor which came with a Zenith carburetor which needed some work (GAV would not turn). This carburetor still worked and ran the motor acceptably. In the mean time I purchased a used/rebuilt carburetor (Zenith 2, Black in attached Photo) off Ebay and it worked fine.

Disclaimer: My first time at rebuilding a Zenith. I cleaned everything, purchased all new jets, GAV, and gaskets. I installed on my good running car with bad results, very rich and black smoke out the tail pipe.

I experimented by swapping the bottom assemblies (My original rebuilt Zenith top and Zenith 2 bottom) and low and behold the car ran great, thus my conclusion was my lower half rebuild was not quite right. Upon investigation of the two lower halves I see some differences and was hoping you experts might chime in here. I believe the carburetor that came with the car is an early Zenith 1 double venturi bottom with the cast iron well, with no threads or provision for a brass well. Also note the positions of the main and cap jets for the double venturi, and the small bowl vent hole.

The attached photo shows both bottom halves; the Zenith (Gray, with the old jets shown), and the Zenith 2 (Black).

The Zenith has NO secondary well, and no place for one (no threaded hole) with a Single Venturi Inside Diameter 0.675” (outside has a recess machined in shoulder) and Cap and Main Jet are on centerline of the carb body. All the jets I removed had numbers, Cap 19, Main 20, Compensator 19, the new jets have NO numbers. As I stated, before I took apart it did work.

The Zenith 2 Single Venturi, Inside Diameter 0.845” and Cap and Main Jet are slightly offset.

Other then that I cannot see any difference; I have read on the Model-A.org site that the Single Venturi should be 0.845. I am not sure what is wrong my lower half yet however it did run before so I hope to get it assembled and running correctly again.

Any help/direction would be great.
Thanks
Scott

Re: Zenith Carburetor Question - Venturi and more
 

The upper carburetor was (is) a double venturi style. The 0.675 venturi might have been made for a special commercial application, but it would not work well with your car. It is OK to use the common 0.845 venturi in place of the original double venturi.

The main jet in the double-venturi carb. (the one with the small hole) originally had a bullet shape to the top. If that is what you have, it might be original. That style is very rare, so be sure to save it if you do not keep it in your carburetor. Repro jets with the bullet top are slightly different in shape.

Re: Zenith Carburetor Question - Venturi and more
 

Re: Zenith Carburetor Question - Venturi and more
 

Re: Zenith Carburetor Question - Venturi and more
 

I have never seen original jets with a bullet shape. A single venturi will not work correctly in a Double Venturi carburetor. The compensator jet, idle jet, and cap jet in a Double Venturi are different than the later jets.

You might refer to the Service Bulletins as they cover the changes to the carburetors, jets, and secondary wells of which there are 3 types.

Ron

Re: Zenith Carburetor Question - Venturi and more
 

The venturi that you show is probably not a Zenith venturi. The flat area is unusual. The original bullet-style jet that I mentioned previously is more gradually tapered than the repro bullet main jets. I have looked at about 19 Holley double venturi carbs and only 4 had the original tapered (bullet) main jet. I don't post pictures so I can't show you the difference.