Zenith carburetor theory

[mrtexas]

Last week I sawed apart several Tilly carbs to make sure I was getting to all the narrow passages. Onward to the Zenith! I'm not going to saw Zeniths apart.

I have found no explanation simple and complete enough for me to understand as to how a Zenith carburetor actually works. I have the Pargeter, Moller, and Biggar booklets. My explanation is refined from the Pargeter explanation with pictures from the Pargeter book. The object of a good carburetor is to maintain the same air/fuel mixture ratio over the expected range of rpm the motor is operated at.


This is the simple carburetor. As engine rpm goes up, the main jet sucks out more fuel to the engine. However, as the rpm increases the air/fuel ratio decreases providing a richer mixture.


The unique feature of the Zenith carburetor is the addition of a secondary well open to the atmosphere to the simple carburetor. This was the basis for the 1905 US patent by Francois Baverey of the French Carburateurs Zenith Co.

The secondary well is open to the atmosphere allowing air as well as fuel to be sucked into the intake thru the cap jet. As the rpm increases the air/fuel ratio increases providing a leaner mixture.

The secondary well increases air/fuel ratio(leaner) as rpm increases counteracting the simple carburetor decrease in air/fuel ratio(richer). The combined result is a more constant air/fuel ratio over all rpms.

Another feature of the secondary well is that it fills at idle and when the throttle is opened, it empties and acts like an accelerator pump.


The 3rd feature is the idle circuit. At idle the throttle plate(butterfly) is closed with all the fuel thru the idle circuit thru a hole in the carburetor throat. The idle jet meters the idle fuel and the air adjusting screw(incorrectly called the idle screw) meters the air(air/fuel ratio.)

However when the throttle is opened, the secondary well is emptied. Then no fuel goes thru the idle circuit, only air. This explains why the air adjusting screw has no impact after the throttle is opened.

GAV is adjusted manually by turning the choke rod. It adds an adjustable amount of fuel thru the cap jet.

[reggiedog]

I'm glade u posted that I'm sure other people will be interested the same as I am. U could post other pic's of that if u want reggiedog

[1931 flamingo]

Last month ( I believe) in the Restorer (MAFCA) there was an EXCELLENT article about the Zenith, on how to clean it, etc, along with excellent pictures, by one of the frequent posters here on the Barn. A must to at least read. JMO
Paul in CT

[Bill Edstrom]

Here is an article by Murray Fahnstock that may help.

[mrtexas]

Originally Posted by Bill Edstrom

Here is an article by Murray Fahnstock that may help.

Good article but not clear and complete enough for me!

"Last month ( I believe) in the Restorer (MAFCA) there was an EXCELLENT article about the Zenith, on how to clean it, etc, along with excellent pictures, by one of the frequent posters here on the Barn. A must to at least read. JMO
Paul in CT"

I looked for back issues of The Restorer and discovered I had let my membership expire. They are mailing out that one to me today!

[broommaker]

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Thanks so much for the drawings of the Zenith!! broommaker
Kentucky, Jim

[30Tudor]

This is the best explanation of how the carb works that I have seen. I have never really understood how the secondary well fits into the picture as it relates to suppling additional air to keep the mixture constant.

Thanks for posting.

[ursus]

This excellent explanation of the function of the secondary well makes me wonder why this feature was not employed in the Model-B carburetors. Some seem to run way too rich - would they have benefited from this earlier design feature?