Model A Zenith Fever Dreams!

Ford Garage

Zenith Carburetor Theory

Jet Sizes & Flow Rates

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The function of a carburetor is to meter, mix, and maintain both air and fuel quantities in a prescribed ratio across a range of engine RPM and operating conditions.

The carburetor's air intake horn, venturi, throttle bore, and jet circuits are all sized in a coordinated manner to suit a specific application by the carburetor designer.

This sizing is done to create and maintain the correct air/fuel mixture ratio for a given engine displacement size and operating speed range, as well as to accommodate for elevation, fuel type, and fuel quality.

Cut to the Chase:

Read on, for an In-depth Discussion:

The Model A carburetor was designed and developed for Ford by Zenith-Detroit, but was supplied in production by both Zenith and Holley (under license from Zenith). According to archive research conducted by George De Angelis in the 1970's, Zenith supplied about two-thirds of Ford's requirements, with Holley supplying the rest.

In my own experience in the Midwest, Holley seems to outnumber Zenith slightly, perhaps 60/40.

Holley-built carburetors and castings are usually marked with a Holley 'H' identification cast inside the bowl on each casting as well as markings on some other component parts, and also usually carry either a Zenith-2 designation cast on the exterior of the float bowl, or a raised 'H' cast on the exterior of the upper and lower castings, at the Venturi. Additionally, Holley upper castings are not through-drilled on the inboard side for the throttle shaft.

All the component parts from both Zenith and Holley carburetors are interchangeable and can be freely mixed and matched in practice.

Typical Jet Numbering:

All original Model A and B Ford Zenith and Holley carburetor jets had the nominal orifice size number stamped somewhere on the jet, making identification easy. No reproduction jets have had the jet numbers stamped on them.

In the March 1930 illustration above, the numbers stamped on the jets are just typical examples. There were other number sizes used for each, e.g. '19', 19.5', '20', etc.

Jets were all machined and manufactured to dimensional requirements, however, the jets also had water flow test performance requirements which also had to be met, as also indicated on the part detail drawings.

Jet Flow Water Test Pressures:

Per the original 1928 Zenith and Ford jet detail drawings, a 1 Meter (39.4 inches) head pressure of water was initially used to specify the jet flow rate test performance.

By mid 1928, Ford and many Zenith jet drawings were revised to reference a head pressure of 37-1/4 inches of water (1.35 psi) to specify the flow rate performance. The origin or significance of the change to a 37-1/4 inches test dimension is not clear.

The detail drawing example above from late 1931 is fairly typical of most Model A and B jet drawings. This particular drawing also indicates that at that time jets were all to be 100% flow tested.

Jet Flow Water Test Rates:

The various specifications reported for each jet in the tables below include water flow test rates and/or orifice sizes. Flow rates are expressed in cubic centimeters/minute (cc/min), which is equivalent to milliliters/minute (ml/min). Orifice sizes are typically stated in either decimal inches or Number Drill size, or occasionally as decimal millimeters.

The tables below also contain many specified size dimensions of orifices taken from original drawings, however, keep in mind that those are initial manufacturing and machining dimensions from Ford and Zenith drawings. The actual performance specified was a water flow test requirement, not a dimensional requirement.

You may also notice that some original components with the same size numbers or orifice diameters have different flow test requirements. This is based on the flow of the fuel through the differing tube geometries and lead-ins to the orifice, as well as the external air pressures acting on the fuel supply and the orifice.

Some components such as the Compensator Jet and Power Jet only flow fuel based on the direct fuel mass gravity and atmospheric pressure acting on them in the float bowl, and only indirectly according to engine suction.

Other jets like the Main, Cap, and Idle are acted on by manifold or venturi suction, effectively lifting a mass of fuel through the jet, and opposed by the gravity force of the fuel itself. This effect of suction versus atmospheric pressure results in different actual fuel flow rates through like-sized orifices in a running engine as well.

The specified water (not fuel) test flow rates are just the standardized water flow laboratory test conditions, and enable the ability to correlate physical jet/flow changes to observed changes in actual running engine test performance.

You can make comparisons and conclusions from the data below, and establish your own personal flow rate targets. Hopefully this information will be helpful in guiding your carburetor rebuilding and tuning activities.

1928-31 Model A Zenith Carburetor ~ Jet Sizes & Water Flow Rates

Summarized below are the jet flow specifications of original Ford Zenith carburetor configurations (shown in green or yellow in the first column), as well as some of the current and relevant published flow numbers recommended by various Model A Zenith carburetor rebuilders and experts in the hobby.

Water flow rates are expressed in cubic centimeters/minute.
(= milliliters/minute, also = grams H2O/minute)

Engine Air Consumption Reference

The table above shows the amount of air consumption for different displacement engines at different RPMs. This info is from Zenith's early 1930's Carburetor Technical Service Manual.

The Model A and B are both 200.5 Cubic inches Displacement at stock 3.875" bore, and 214 CID at 4.000" bore (+0.125" max oversize).

Many 'Carburetor' Problems Are Actually Electrical

Electrical and Ignition System:

1. Model A battery cable is laying/rubbing across the top of the brake pedal rod.
   Worn cable insulation can cause an intermittent short-to-ground during braking, and can cause momentary ignition failure and stalling when applying the brakes.

2. Spark Plugs are dirty or have incorrect gap.
   Clean and set spark plug gaps equally to 0.035" max (no less than 0.030").

3. Distributor contact points are dirty or have incorrect gap.
   Clean and set ignition point gap to 0.022" on new points, 0.020" on used points.

4. Wire between the lower distributor plate and the distributor contact point stud (under the top plate) is frayed and can cause an intermittent short-to-ground during spark advance movement.
   Remove the distributor top plate and check the condition of the wire.
   See linked page below for recommended wiring and replacement.
   Replace wire and retime the ignition.

5. Ignition spark is not correctly timed.
   (Re)time the distributor contact points according to the Model A (or Model B if appropriate) Ford Instruction Book or Ford Service Bulletins instructions, after setting the contact point gap and/or after inspecting the internal distributer wiring.

6. Coil to Distributor Cap high tension wire is not seated properly.
   Check condition and fit of coil wire terminals in the coil nipple and in the distributor cap.
   Use a solid conductor high tension coil wire, not carbon threaded.
   Solder the brass wire terminals to each end of the coil wire conductor for assurance.

7. Distributor cap carbon button is worn, damaged, or missing.
   Check condition of the carbon button in the distributor cap.

8. Distributor rotor top contact spring is not contacting the carbon button consistently.
   Check to ensure some spring preload on cap before attaching distributor cap and body metal spring clips.

9. Ignition coil is not wired with correct polarity.
   For a positive ground system, the negative (-) coil terminal is connected to either of the two terminal box brass studs.
   The positive (+) coil terminal wire goes to the ignition switch, through the switch, and then to the distributor movable contact point. (and then through the points to ground)
   Coil primary ground path is completed when the ignition points in the distributor close, allowing the coil to energize.
   The coil high tension secondary fires when the contact points in the distributor break 'open'.

10. Ignition coil may be weak or failing.
    A strong ignition coil produces a bright blue high voltage spark.
    A weak low voltage yellow spark indicates a failing coil, poor ground, or excessive resistance.

11. A weak or failing condenser (capacitor) can also cause intermittent poor running.
    A condenser will sometimes test okay cold, but begin failing as the engine heats up.
    Replace the condenser with a new 0.25 microfarad 'burnout-proof' condenser from Snyders.

12. Throw away your timing lights, pulley indicators, clever cam wrenches, over-thought timing gadgets, and magic beans!

First Things First!

Fuel System Basics:

1. Did you verify you have clean fuel, in a clean vented tank, and free flow out the fuel line at the carburetor while the tank valve is open? It should piss like a race horse, technically speaking!
   Okay, let's find out why it dies on stopping, or won't idle properly!

2. First, verify the fuel line is not extended too far into the upper carburetor casting, contacting the Fuel Strainer and limiting flow volume into the carburetor. The compression ferrule ('olive') should be no more than 1/8" from the end of the fuel line tubing.

3. Verify the Fuel Strainer screen (inside the non-sidebowl carburetor) is not clogged with rust or debris, limiting flow volume to the float fuel valve and carburetor bowl.

4. Verify that the throttle shaft lever on the carburetor has a full range of travel when actuated by the accelerator control and the hand throttle.
   Ensure that the carburetor throttle shaft lever fully returns to the stop pin position, and can be opened to the wide-open throttle position when installed on the engine and in the car.

5. Ensure the use of the correct length throttle control rod.
   Loosening the clamp and rotating the mast jacket on a 2-Tooth steering column will allow some adjustment.
   It may be necessary to bend the long forged steel arm of the throttle control assembly where the throttle control rod attaches.

6. Verify that the Fuel Level (not the float height) in the float bowl is correct.
   Measure and set the fuel level 5/8" ± 1/32" below the fuel bowl gasket surface using an externally visible sight gauge mounted in the (3/8-24 thread) drain plug hole. Use fuel or mineral spirits, not water, to measure the fuel level. Vary the thickness of the fuel valve gaskets to change the float height and fuel level. Do not bend the float in any way.

Won't Idle Properly and/or Stalls When Stopping

Idling Fuel System:

1. Idle RPM is set too low (<350) to recover after the (open) throttle is suddenly released/closed.

2. Idle RPM is set too high (>550) and the engine is not actually running on the idle circuit or at idle speed (~450 RPM idle target).
   The throttle shaft lever adjustment screw and plate are set too far open, and the engine is actually "fast idling" on the fuel and air from the Cap Jet and through the Venturi, not the Idle Jet and Air Mixture Screw (Needle) circuit.
   In this fast idle condition, the GAV will have some effect on the (fast) 'idle' quality.
   Though the engine is running, this is not the normal or intended idle RPM condition.

3. The Idle Air Mixture Screw (Needle) is improperly adjusted and the idle mixture is too lean (too much air/too little fuel).

4. A too-high idle RPM can also cause a worn or low-compression engine to backfire through the exhaust after turning off the ignition.
   The engine continues to draw fuel/air mixture through the carburetor after the ignition is off, yet before the engine stops rolling.
   The unburned mixture can then ignite explosively inside the hot muffler.
   Set the idle speed between 450 and 550 RPM.

5. The fit of the closed-position of the throttle plate in the casting bore is poorly aligned.
   Back off the throttle shaft lever stop screw, loosen two throttle plate screws, and reset the fit of the plate in the bore in the fully closed position.
   Retighten the throttle plate screws.
   Re-establish the correct idle speed with the idle stop screw on the throttle shaft lever.

6. A vacuum leak to the atmosphere is present at either/both ends of the throttle shaft in its fit to the shaft bores in the upper casting.

7. A vacuum leak to the atmosphere is present at the carburetor-to-manifold flanges or gasket.

8. The Idle Jet siphon tube is cracked or damaged, or has a loose fit to its threaded upper fitting.
   These conditions can cause a vacuum leak, making it unable to draw fuel up the tube.

9. The Idle Jet orifice/flow is too small/lean, especially for ethanol mix fuels.
   Ethanol-blended fuels have lower energy per volume compared to gasoline, and need a larger idle fuel flow rate.

10. The Idle Jet orifice is obstructed with rust or dirt.

11. The Idle Fuel aperture (also called the 'Priming Hole') in the upper casting throttle bore is obstructed with rust or dirt or insect nests.

12. The Idle Fuel and Air passages in the upper casting are obstructed with rust, dirt, insect nests, spider webs, or cocoons.

13. The two small orifices in the bottom of the brass Secondary Well (on Model A) are partially plugged with rust or dirt.
    This blockage at the bottom of the Secondary Well causes the idle jet fuel supply to run dry.
    The Well must be removed from the iron casting to properly clean the Well and the casting itself.

14. The Compensator Jet orifice is obstructed, starving both the Secondary/Idle Well and the Cap Jet.
    In that case, the idle well is only supplied by fuel flowing past an open GAV seat, back-feeding the Secondary/Idle Well.

15. The brass Secondary Well (on Model A) is contacting or obstructing the Compensator Jet.
    Use the 3rd (newest) Ford brass Secondary Well design to ensure clearance for Compensator Jet flow into the Secondary/Idle Well. See the Secondary Well link in the list below. The current reproduction Secondary Well is correct for use.

16. A vacuum leak is present at any of the various vacuum line connections of the wiper motor.

17. A vacuum leak is present at the intake manifold-to-block gasket, or intake manifold-to-carburetor gasket.

No Power at Running Speed and/or Speed is Limited

Open-Throttle Fuel System:

1. Ensure the Main Jet orifice is not obstructed with rust or dirt.

2. Ensure the float bowl passage to the Main Jet is not obstructed with rust or dirt.

3. Ensure the float bowl passage to the GAV is not obstructed with rust or dirt.

4. Ensure the Compensator Jet orifice is not obstructed, starving both the Secondary/Idle Well and the Cap Jet.
   In that case, the idle well is only supplied by fuel flowing past an open GAV seat, back-feeding the Secondary/Idle Well.

5. Ensure the Cap Jet orifice is not obstructed with rust or dirt.

6. Ensure the lower casting passage between the GAV and Secondary Well/Cap Jet is not obstructed with rust or dirt.

7. Verify that an actual Compensator Jet is installed in the float bowl, and is not mixed up with a GAV Needle Seat in its place (common mistake, very similar appearance).

8. Verify that the Compensator Jet in the float bowl is not contacting the Brass Secondary Well, blocking flow through the Compensator Jet.
   There were primarily three different brass wells, and the current reproduction will work in all carburetor iterations.

9. Ensure that the tip of the GAV Needle is not jammed into the brass GAV Needle Seat, causing the obstructed seat to rotate with the needle (unscrewing from the casting) as the choke rod is rotated. (not an unheard of condition).
   A jammed condition will defeat the operation of the GAV circuit from augmenting the Cap Jet fuel.
   Some lower castings use an integral iron seat which cannot jam or unscrew.

10. Verify that an actual GAV Needle Seat is installed in the base of the lower casting GAV bore, and is not mixed up with a Compensator Jet in its place (common mistake, same threads and very similar appearance).

A Word About Rebuilders, YouTube Videos, Social Media, etc.

Be quite wary of unverified advice and peanut gallery comments found online regarding Model A Ford Zenith carburetor theory of operation, performance, and rebuilding! There is a vast amount of incorrect and ill-advised Zenith info and opinions being peddled on internet message boards, YouTube videos, social media posts, and eBay listings.

Those postings may be well-intentioned but are very often factually wrong, mistaken, misleading, and in some cases border on nonsense. Ford Garage makes a very strong effort to stay centered on verified facts from primary sources, and welcomes constructive feedback on potential errors or omissions, and on items deserving further discussion and clarification.

The vast majority of Zenith information presented here on Ford Garage comes directly from original era Ford and Zenith detail part drawings and 1930's OEM catalog information, Zenith Service Manual publications from the early 1930's, from various Dykes carburetor books and publications from the 1930's, and from an extensive collection of Model A and B Ford Zenith carburetors and Zenith rebuilding experience, as well as from discussion and collaboration with a few knowledgeable Zenith experts.

If you are looking online for expert knowledge, insight, and advice on the Model A and B Ford Zenith carburetors, as well as quality Marvel and Model B Zenith new parts and services, new flow-tested Zenith jet sets, and also nicely designed fuel level sight gauges, contact David Renner at Renner's Corner in Manchester, Michigan, linked in the list below.

If you are looking for a very experienced Zenith carburetor rebuilder, you should consider Steve Becker at Bert's Model A Store in Denver, Colorado for knowledgable and reliable work, according to many testimonials.

Based on numerous buyer 'testimonials' posted online in Model A forums, eBay is NOT the place to buy a properly rebuilt Zenith, despite the apparent sales success of certain eBay sellers/rebuilders. YMMV.

If you are looking for good Zenith rebuilding books, all of the 'classic' books and club magazine articles from the Model A hobby authors of the 1960's-90's contain many factual errors and incorrect conclusions, and can easily lead you astray if you accept everything at face value. A combination of Gordon Biggar/Rex Reheis' (OOP) and Steve Pargeter's (Version 8) most recent books offer the most accurate and useful Zenith hobbyist information published in the last 50 years.

Additionally, Rex's coverage of the Model B Zenith carburetor is about the best you will be able to find in recent print for that specific carburetor. Also, David Renner and George DeAngelis authored a number of very good Model B Zenith articles in Secrets of Speed magazine in the 1990-2000's.

In MARC's and MAFCA's bi-monthly club magazine publications, Steve Schmauch's recent (2017-2023) series of Model A Zenith articles in The Restorer magazine on Zenith and Holley casting differences and identification are very comprehensive, insightful, and helpful. Many other Model A Zenith carburetor magazine articles, past and present, have been an unreliable mixture of wheat and chaff, requiring your own knowledge, capability, and expertise to distinguish between the two.

The MARC/MAFCA jointly-published Restoration Guidelines & Judging Standards manual contains the final word on carburetor details and authenticity for judging purposes, and is an invaluable resource for people who seek to achieve as much authenticity and correctness of parts and appearances as possible.

The Zenith web sites of Paul Modlin and Chris Pelikan (now maintained by Bert's) are both very helpful and are both linked in the list below.

More related information on Ford Garage:

1. For more Model A & B related information, use the Site Search box at the top or bottom of this page.
2. Model A Zenith Carburetor Theory of Operation
3. Model A Zenith Carburetor Numbering
4. Model A Zenith Carburetor A-9545 Secondary Well Variations
5. Model A Zenith Carburetor Float Mass and Performance Details
6. Model A Zenith Carburetor GAV Wrench
7. Model B & 46 Zenith Carburetor Parts Catalog Illustrations
8. Model B & 46 Zenith Carburetor Assembly Drawings
9. Model B & 46 Zenith Carburetor Float Mass and Performance Details
10. Model B & 46 Zenith Float Fuel Valve Orifice Details
11. Model B & 46 Zenith Carburetor Power Jet Circuit
12. Model B & 46 Zenith Carburetor Power Jet Tube
13. Model B & 46 Zenith Carburetor Power Jet Tube Instructions
14. Model B & 46 Zenith Carburetor Catalog Data
15. Model B & 46 NOS Zenith Carburetor
16. Model B & 46 Zenith Carburetor Upper Casting Variations
17. Model A & B & V8 Spark and Throttle Control Rods
18. Model A & B & V8 Renner's Corner, Quality Model A & B Zenith and Marvel Carburetor Parts and Advice
19. Model A Paul Modlin's Zenith Carburetor Animations on Model A Basics.com
20. Model A Chris Pelikan's Zenith Carburetor MasterClass on Model-A.org
21. Model A & B Throttle Shaft Bushing Install ~ Randall Strickland on YouTube
22. Model A Distributor Lower Plate Wiring Solution
23. Model A & B & V8 Vacuum Gauge Tuning from ARSCO
24. Model A & B & V8 World War II Training Film: Automotive Troubleshooting. 1942 on YouTube
25. Model A & B & V8 World War II Training Film #497: Mechanics, Champion Ignition and Spark Plug. c. 1941-1945 on YouTube