Zenith Carburetor Question - Venturi and more

[scokeing]

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