Fuel level 5/8 inch where did this figure originate?

[johnbuckley]

When was the first recommendation to set the fuel level on Zenith at 5/8 inch below the top of the lower body ? Does anybody have an original service manual showing this figure? ( I know many people think the level should be set be higher or lower....).

[Joe K]

I can't say positively, but I think it is driven by the height of the jet(s) in the venturi.

Jets appearing below this level will leak, and jets appearing above this level require an inrush of air to "lift" the fuel. If the jet is TOO high, a larger airflow is required to lift fuel and it causes the carburetor to run "lean." There is some reverse compensation for this in the GAV, which introduces a variable resistance to the flow of gasoline.

One speculates that the jet is situated slightly high (to prevent dripping), the orifice is sized slightly "large" (to assure a surplus of gasoline) and the GAV is where the adjustment is made.

If one travels at a constant speed, one finds a spot on the GAV that is "just right." A different speed yields a different "sweet spot."

Well, that's my carburetor, anyway.

Joe K

[johnbuckley]

yes Joe , That's the rationale behind the recommendation, but where in print did the figure first appear? . I can't find it in the service manuals was it in a 1920s/30s Ford manual, or a Zenith sheet of that time?

[Steve Plucker]

John,

You raise an excellent question.

Of all the "Carburetor Restoration Information" out there that have been written of which I have 4 major books on, the earliest mention of the "fuel level" when restoring the carburetor is in the 1984 issue of Paul Moller's "Model A Ford Restoration and Maintenance Handbook, Vol. 2". HOWEVER...Paul tells us that that level should be for Zenith and Holley carbs "1/2 inch below the gasket between the upper and lower bodies...Tillotson is 3/8 inch and Zenith Model B is 5/8 inch". Just where he came up with these figures is unknown.

Contained in the other three articles: Tom Endy's "Model A Ford Zenith Carburetors"; Gordon Biggar's "Restoring The Model A and B Carburetors...The Rex Reheis Way"; and Steve Pargeter's "Zenith Model A Carburetor Restoration Guidelines" all tell us that the figure is 5/8 inch.

Pargeter's book (Version 3) is the only book that lists reference material...32 to be exact.

But you are right...the figure of 5/8 inch is not found in any of the Ford Service Bulletins or Ford Service Letters that I have seen unless we have missed it somewhere.

I would be interested myself to know just where this figure came from originally.

Pluck

[Bob Bidonde]

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Ditto re Paul Moller, and the context of his words suggest he determined the 5/8" float setting from experience, "The surface of the float as it rests on the float valve should be parallel to the machined edge of the upper body. Normally this measures out to 5/8"."


It has been my experience that the 5/8" float setting is an iteration starting point. There are a few factors to contend with especially the inclination of the carburetor as installed on the engine.