Model A

Zenith Carburetor

A-9545 Secondary Well Variations

The brass Secondary Well A-9545 was released by Engineering June 1, 1928 with the initial design release of the Single Venturi carburetor.

The Secondary Well is located within (and is supplied fuel by) the larger surrounding Compensator Well, which is machined directly in the lower iron casting. The iron Compensator Well is supplied fuel by the Compensator Jet (metering orifice) located in the lower rear of the float bowl.

The Secondary Well is the fuel well in which the Idle Jet siphon tube is suspended. This Well feeds the Idle Jet during idle operation, as well as providing some fuel transfer effect to the Cap Jet during the initial transition from idle speed to acceleration when coming off idle.

During off-idle acceleration and open throttle operation, the brass Idle/Secondary Well maintains a partial fuel level available for use by the Idle Jet whenever the Throttle Plate is suddenly closed, or quickly returned to the idle stop position (during braking for example), and prevents stalling during the return-to-idle fuel circuit transition.

The two small metering holes in the bottom of the brass Secondary Well control the rate at which fuel for the Idle Jet can enter from the surrounding machined iron Compensator Well (into which the Secondary Well is screwed).

The one small hole near the top of the brass Secondary Well is a vent to atmosphere, also called a vacuum breaker, and prevents a vacuum from being pulled on the Compensator Jet by the Venturi (via the Cap Jet). Thus the fuel in the float bowl passing through the Compensator Jet only has atmospheric pressure acting upon it.

The fuel transfer rate into/out of the brass Secondary Well is metered in order to limit back-feeding the Cap Jet from the Well during acceleration. The purpose is to retain some fuel in the Well in case of sudden return-to-idle conditions, such as when 'blipping' the throttle controls, or during and idle/acceleration/(braking)/idle sequence.

The primary fuel delivered to the Cap Jet during acceleration is provided directly by the fuel from the float bowl, passing through the Compensator Jet, into the iron Compensator Well, around the outside of the Secondary Well, and on to the Cap Jet. This circuit provides the smooth transition effect between idle and acceleration.

There was no brass secondary well in the earlier 1928 Zenith Double Venturi carburetor design. There was only an Idle/Compensator Well feature, machined directly into the lower iron casting.

The action of the die cast zinc Double Venturi itself provided the transition from idle to acceleration, using the fuel supplied through the Compensator Jet in the float bowl, and discharged by the Cap Jet into the smaller Venturi air stream.

Notably also, The early Double Venturi Idle Jet itself was 5/64 inch longer than the typical later Single Venturi Idle Jet due to the difference in the Well depths for the Siphon Tube.

The illustration below is from the June 1930 Canadian Service Bulletins, and shows the design change progression of the Model A Zenith carburetor A-9545 Secondary Well.

Note that the Ford Figure 24 illustration above references the undercut/groove alignment with the "Cap Jet hole". This is a typographic/labeling error in the illustration, and the groove actually aligns with the threaded Compensator Jet hole in the float bowl.

Changes were made to the brass Secondary Well to improve fuel flow around the outside of the well from the Compensator Jet in the float bowl, and onward to the Cap Jet. The earliest brass well designs could allow contact between the Compensator Jet and the outside of the brass well, and obstruct/restrict fuel flow through the Compensator Jet.

The clearance groove/undercut in the outside of the Secondary Well was moved upward to better align with the drilled passageways in the iron casting where the Compensator Jet protrudes, and to prevent any possible obstruction of the Compensator Jet.

Style 1 is the original design with the groove towards the bottom.
Style 2 is an intermediate/transition design/rework showing the addition of an upper groove to the Style 1 design.
Style 3 is the final design, having the lower groove removed, and retaining only the upper groove position for improved gas flow.

Additionally there exists what I term a Style 3B and 3C. They are similar to the common and typical Style 3 (3A), except not having the undercut relief at the top of the well around the vacuum breaker vent hole. Additionally, Style 3C also has a noticeably longer threaded length at the top, but is rarely encountered. It is also possible that 3B and 3C are era non-Ford replacement parts.

These changes improved fuel flow from the float bowl, through the Compensator Jet, around the outside of the Secondary Well, and subsequently to the Cap Jet.

It is recommended to only use a Style 2 or Style 3 Secondary Well in your Zenith carburetor, and to confirm that the Compensator Jet flow is not physically obstructed by the Secondary Well itself.

As a point of interest, the threads of the Secondary Well are 3/8-24 NF2, and the inside diameter is 5 mm, or about a #8 drill (0.1990").

Note: The brass Secondary Well should be fully seated in the lower bowl casting for two important reasons.

First, the relief groove in the outer diameter of the Secondary Well must align with the Compensator Jet, and this barely occurs even when the Well is fully seated. If there is not proper alignment and clearance to the Compensator Jet, the downstream Cap and Idle Jets can be somewhat starved for fuel supply by the obstruction.

Second, if the Secondary Well is not seated, the Idle Jet siphon tube can contact the inside bottom of the Well and directly obstruct the fuel flow into the Idle Jet.

More related information on Ford Garage:

For more Model A & B related information, use the Site Search box at the top or bottom of this page.
Model A Zenith Carburetor Theory of Operation
Model A Zenith Carburetor Numbering
Model A & B Carburetor Jet Sizes & Flow Rates
Model A Carburetor Float Mass
Model A Zenith Carburetor GAV Wrench

July 2012