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Model A & B

Ford Garage

Zenith Carburetor Jet Sizes & Flow Rates

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

The Model A carburetor was designed and developed for Ford by Zenith, 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.

Holley-built carburetors and castings are usually marked with Holley identification inside the bowl on each casting as well as some other parts, and also usually carry a Zenith-2 designation on the exterior of the float bowl.

Single Venturi illustration shows the open-choke, closed-throttle idle condition.
'A' is the Venturi. 'B' is the Secondary/Idle Well.

Theory of Operation

Both Model A and B Zenith carburetors are updraft design, and are entirely vacuum and fuel-gravity operated. There are no pistons, plungers, pumps, actuators, or other active features in their operation aside from the throttle plate, choke plate, and the GAV knob.

The Model A Zenith carburetor has three jet circuits which consist of an Idle Jet circuit, a Compensator Jet (measuring/metering orifice) circuit, and a Main Jet circuit. The Cap Jet (so-called 'jet') is not a measuring or metering jet circuit per se, though it is acted on by Venturi suction. The Venturi was also sometimes referred to as a 'Diffuser' in Ford and Zenith literature.

Airflow into the carburetor is controlled by the Air Shutter (Choke Plate) position in the lower casting. The Choke Plate is normally open, except when applying the choke during cold starting. Zenith company literature often referred to the Choke as the 'strangler'.

Airflow through the carburetor is subsequently controlled by the Throttle Plate position in the upper casting. The flow of the air/fuel mixture through the Venturi and into the throttle bore increases as the Throttle Plate opens, resulting in increased engine speed. The size of the Venturi determines the maximum volume of air which can be passed through the carburetor.

The airflow through the Venturi during off-idle and open-throttle conditions determines the amount of suction applied to the Main and Cap Jets at the Venturi.

The limited airflow past a near-closed Throttle Plate determines the amount of engine manifold suction applied to the idle mixture discharge orifice (Zenith called it the Priming Hole) at the edge of the Throttle Plate in the throttle bore. The Idle Stop Screw on the Throttle Lever sets the near-closed Throttle Plate idle position, and thus the resulting idle rpm.

The Idle Jet assembly flows fuel according to the engine manifold vacuum above the Throttle Plate only when the Throttle Plate is nearly closed. The Idle Jet ceases to flow fuel as the Throttle Plate is opened further, and as manifold vacuum decreases on the Priming Hole in the throttle bore at the edge of the Throttle Plate.

The air used in the idle air mixture is made up of the air admitted through the bowl vent by the air mixture screw needle, combined with the larger volume air flowing past the nearly-closed Throttle Plate in the upper casting bore. The primary purpose of the mixture screw needle is to regulate the suction applied to the fuel in the Idle Jet assembly itself.

The Idle Air Mixture Screw (Needle) meters and mixes outside air (via the carburetor bowl vent) with the fuel drawn through the Idle Jet assembly (lifted through its siphon tube suspended in the Idle/Secondary Well), as well as regulating the actual suction applied to the Idle Jet and siphon tube assembly by the manifold vacuum at the Priming Hole. The Air Mixture Screw Needle is effectively a controlled air leak in the idle circuit's fuel drinking straw (the Idle Jet assembly).

The actual Idle Jet assembly Orifice is located at the top of the brass Idle Jet assembly (which is screwed into the upper casting). The Idle Jet assembly also has a siphon tube which extends down into the brass Secondary Well in the lower casting (or into the iron Compensator/Idle Well in the case of a 1928 double venturi).

That idle air-fuel mixture is discharged through the Priming Hole (idle discharge orifice) in the upper iron casting throttle bore (above the Venturi) during near-closed throttle conditions, and at engine speeds less than 550 RPM.

The Secondary/Compensator/Idle Well is more than half full of fuel when the engine is idling. This provides a reserve for acceleration when the throttle is opened off-idle. The fuel is diverted to and through the Cap Jet by the increasing suction of the Venturi (at engine speeds greater than 550 RPM).
All Double Venturi Model A carburetors had only a machined iron Compensator/Idle Well for the Idle Jet siphon tube, but no Secondary Well in the lower iron casting.

In the case of the Double Venturi carburetor (which did not have a brass Secondary Well), the higher air flow speed between the inner and outer Venturis likely changed the response of the Cap Jet favorably at off-idle conditions without using a separate fuel reserve in a Secondary Well for the return-to-idle circuit transition.

Ford and Zenith ended production of the Double Venturi design beginning in June 1928 due to a patent infringement lawsuit from Stromberg, which Ford later lost. Stromberg did not have any Model A business at that time, but later became a Ford supplier using their double venturi designs/patents on flathead V8 carburetors in the later 1930's.
All Single Venturi carburetor lower iron castings have a machined and threaded Compensator Well which contains a separate brass Secondary Well for the Idle Jet siphon tube. The Idle Well normally maintains about a 1-3/8" level of fuel.

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

The Compensator (Comp) Jet (actually a measuring/metering orifice) is located in the float bowl, and its flow rate into the Secondary/Compensator/Idle Well is constant, governed only by atmospheric pressure, and is independent of Throttle Plate position or Venturi suction across a changing range of engine speeds.

The Compensator Jet maximum fuel flow is the same at all speeds, but its resulting effect is most noticeable at low speeds, or when under a load, or during acceleration.

The fuel metered by the Compensator Jet at idle (<550 RPM) supplies the Idle circuit via the Secondary Well. As the suction at the Venturi increases with increased throttle opening and engine speed, the fuel which passes through the Compensator is biased away from the idle circuit, and is discharged through the Cap Jet and into the Venturi air flow.

The Compensator Jet's maximum fuel flow is limited by its orifice size and atmospheric pressure in the float bowl, not by the manifold vacuum on the Idle circuit or by the applied venturi suction on the Cap Jet. This is due to a small drilled vacuum breaker (vent hole) in the top of the Secondary Well in a Model A, or in the case of a Model B a small drilled hole in the threaded barrel of the Idle Jet.

The Gas Adjusting Valve (GAV) is an adjustable needle and seat which meters additional fuel directly from the float bowl into the well supplying the Cap Jet, but bypasses the Compensator Jet orifice restriction. The added fuel flow rate is manually controlled by the driver of the vehicle using the needle control knob at the head of the choke control rod.

The Cap Jet (so-called 'jet') is more of a discharge tube (outlet orifice) at the Venturi. It does not meter or measure any fuel itself. As the suction at the Venturi increases with throttle opening and engine speed, the Cap Jet combines and discharges all of the fuel previously metered and provided by both the Compensator Jet and the GAV control needle.

The Main Jet is the high speed jet and flows fuel directly from the float bowl according to its orifice size, and according to the direct suction applied at the Venturi. The fuel flow increases as the air flow and suction at the Venturi increases with throttle opening and engine speed.

In a Model A Zenith carburetor, the total fuel delivered in off-idle and open throttle conditions is the sum of the fuel metered by the Compensator Jet to the Cap Jet, plus the fuel supplied by the Main Jet, plus any additional fuel supplied by the GAV.

As-found carburetors are often a mismatch of various Double Venturi and Single Venturi castings and parts from 90 years of modifications, repairs, and rebuilds. Additionally, mis-matched parts from both Zenith and Holley are now often found together.



The Model B Zenith carburetor has three similar circuits to those described above, as well as an added Power Jet (measuring/metering orifice) circuit. The Power Jet is located in the float bowl and meters additional fuel during open throttle conditions and position, and according to engine manifold suction in the throttle bore above the Venturi (not according to airflow suction through the Venturi).

Fuel flows through the Power Jet into a drilled Power Jet Tube Well in the lower casting, up through a Power Jet Siphon Tube, and through a #60 (0.038 - 0.042 inch) drilled Priming Hole located just above the Venturi and at the edge of the Throttle Plate in the upper casting throttle bore.

The suction applied to the Power Jet circuit is controlled by a slot machined into the inboard end of the brass Throttle Shaft, and which together with the shaft bore in the upper casting acts as a vacuum valve based on throttle lever/shaft position.

The maximum fuel flow through the Power Jet is determined only by its orifice size and atmospheric pressure in the float bowl. This is due to a drilled #60 vacuum breaker (vent hole) in the upper iron casting, above the Power Jet Siphon Tube Well. The Power Jet Siphon Tube Well is machined in the lower iron casting.

In a Model B Zenith carburetor, the total fuel delivered in off-idle and open throttle conditions is the sum of the fuel metered by the Compensator Jet to the Cap Jet, plus the fuel supplied by the Main Jet, plus any additional fuel supplied by the GAV, plus additional fuel provided by the Power Jet circuit.

What's up with the GAV, anyway?

The Model A and B Ford Zenith carburetors were designed with a distinctive feature, which no other era carburetors had. That feature is the driver-controlled Gas Adjusting Valve, or GAV. The GAV is the adjustable mixture knob inside the car, which also operates the choke control.

Ford put the GAV knob inside the car so that the driver could manually vary the richness when running by adding supplemental fuel to the airstream of the carburetor (added to/through the Cap Jet). The combined air-fuel mixture drawn into the engine through the Venturi is controlled by the Throttle Plate position, via the gas/accelerator pedal (throttle control assembly).

The GAV augments the fuel which the carburetor already automatically supplies. This operator-controlled extra fuel can aid cold starting and warm-up, heavy load conditions, or make up for poor quality fuel, for example.

The Model A Zenith carburetor does not have a fast idle cam or solenoid acting on the throttle shaft lever, or have a thermostatic choke control, unlike typical 1940's-1980's carburetors. There is also no accelerator pump in the carburetor to squirt an extra shot of fuel. Pumping the accelerator pedal on a Model A has no effect when starting the engine, other than to vary the throttle plate position (further) off-idle.

The choke control, GAV knob, and hand throttle on the Model A can all be used together by the driver to set a comparably richer and faster idle condition during cold weather starting and warm-up, as the need arises.

After the engine is started and warmed up, the choke plate can be fully opened, the idle speed reduced (by hand throttle), and any added fuel from the GAV can be reduced or shut off.


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.

The number stamped on the jet is the orifice size, as expressed in 0.05 millimeter increments.

For example, a jet marked '18' has an orifice diameter of (18)*(0.05 mm) = 0.9 mm (= 0.0354 inches).

Jets were all machined and manufactured to dimensional requirements, however, the jets also had flow test performance requirements indicated on the part drawing, which they also had to meet.



Jet Water Flow Test Pressures:

Per the original 1928 Zenith and Ford jet detail drawings, a 1 Meter (39.4") head pressure of water was initially used to specify the 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. It is not clear the origin or significance of the 37-1/4" test value.

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 to be 100% flow tested.

To muddy the water a bit further (pun intended), a 36 inches head pressure of water (1.30 psi) has long been used as the reference test pressure in nearly all articles and books published by Model A/B hobbyists over the past 50+ years. I don't know what brought about these changes in head pressures for testing, but it is something to be aware of.

The flow differences through a jet will be roughly proportional to the square root of the head pressures, therefore the flow difference between a 36 inches head and a 37-1/4 inches head of water will be under 2%.

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.

In actual vehicle use in a running engine, the flow rate through the jet is also affected by whether engine vacuum or atmospheric pressure is acting on the fuel passing through 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 them, against 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 changes in actual running engine test performance.

The water flow test values do not have any direct correlation to the actual fuel flow rates through the jets during actual engine operating conditions.

The actual operating fuel flow rates in the carburetor of a running engine are based on the particular pressure acting on the fuel at each of those various jets, i.e. atmospheric pressure, manifold vacuum, or venturi suction.
At the end of the day, what matters most is whether the running engine performs to your satisfaction.

Understanding the measured test flow rates through the jets is a way to make quantitative comparisons of performance changes due to jet changes.


If the performance is not satisfactory, knowing the initial jet flow rates gives a basis for further modification of the jet flow rates. Treat the physical size dimensions as additional interesting information.

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

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 published flow numbers recommended by various Model A Zenith carburetor rebuilders and experts in the hobby.

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


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

External
Adjustments
Venturi
A-9586
Throttle Plate
A-9585
Idle Jet
A-9542
(manifold
vacuum)
Main Jet
A-9534
(venturi
suction)
Comp Jet
A-9575
(atmospheric
pressure)
Cap Jet
A-9538
(venturi
suction)
Idle Air Mixture
Screw (Needle)

meters air/vacuum
in the idle mixture
circuit for closed
throttle operation.

Idle Target:
450 ± 100 RPM.


Initial setting to be
1-1/2 turns open
before tuning.

Idle needle angle is
33 degrees included.


Throttle Lever Stop-
Screw
sets the idle
RPM, after the Idle Air
Mixture Screw Needle)
has been tuned.


Gas Adjusting Valve
needle & seat
meters
added fuel to Cap Jet,
direct from float bowl
for start & warm-up.
GAV affects off-idle.
GAV does not affect
idle speed or quality
after warm-up.

Original brass GAV
Seat is Marked '38'
(#49 drill size)
Repros are slightly
smaller in diameter.

GAV needle angle is
30 degrees included.

GAV full flow = 150
@ one turn open*
Double Venturi
(Zenith 30016)
(Zenith 30023)
until approx
September 1928



Single Venturi
(Zenith 30186)
Engineering
drawing release
June 1, 1928
Marked '20'
(Zenith 30141)
until approx
September 1928

Used with
circular-shaped
idle Priming Hole.



Marked
'18-1/2'

(Zenith 30171)
Engineering
drawing release
June 1, 1928

Used with
keyhole-shaped
idle Priming Hole.
Idle Jet fuel
is supplied via
the Comp Jet,
through the two
lower orifices in
Secondary Well.

Throttle position
applies vacuum
on Cap Jet and
diverts fuel from
Secondary Well
& Idle Jet circuit
as throttle opens.
Supply to Main
Jet is direct
and unmetered
from float bowl.

Throttle position
under load
applies vacuum
on Main Jet (via
Venturi) for high
speed operation.
Compensator Jet
(actually orifice)
meters fuel from
float bowl into
Compensator Well
& to the Cap Jet
(and to Idle Jet
via two lower
orifices in the
Secondary Well).

Throttle position
& engine vacuum
does not directly
act on the off-idle
flow through the
Compensator Jet,
but instead acts
on the Cap Jet itself.
Cap Jet supply
is metered by
the Comp Jet,
& is augmented
by the GAV
needle & seat.

Throttle position
off-idle applies
vacuum on Cap
Jet (via Venturi)
for low speed
operation.
Model A Zenith Fuel Level:
The fuel level (not the float height) in the 1928-31 Model A Zenith carburetor float bowl was originally set and regulated at 5/8" ± 1/32" below the fuel bowl gasket surface, per 1932 Ford Service Bulletins page 9, and 1934 Ford Service Bulletins page 227.

Use an external visible sight gauge mounted to the drain plug hole to measure the fuel level in the float bowl of an installed carburetor, or use a glass jar attached to the upper casting in place of the lower bowl casting. Use fuel or mineral spirits, not water, to set the fuel level (float height).

It should also be noted that the carburetor itself is horizontal in the vehicle-installed position, even though the engine is inclined at 3.25 degrees to horizontal. The intake manifold flange is opposite-angled 3.25 degrees to produce the horizontal mounting condition when the carburetor is installed on the engine.
*Note:
Per the original 1928-31 Ford Service Bulletins and Instruction Books, the Model A GAV (Gas Adjusting Valve) should only be opened a maximum of 1/2 turn for starting and warm-up, and not be operated at more than 1/4 turn open thereafter.

In the case of modern recommended jet flow rates, some configurations are sized to run generally leaner than original, but allow greater compensation by using the GAV more liberally.

Original Model A Ford Jet Flow Rates

Flow Rate
Ford Specifications
cc's/min (ml/min)

Thread Size
Venturi
A-9586
Throttle Plate
A-9585

5-40 USF
#4 Oval Head
Idle Jet
A-9542

M5x0.75
10-34 USF
Main Jet
A-9534

M5x0.75
10-34 USF
Comp Jet
A-9575

M5x0.75
10-34 USF
Cap Jet
A-9538

M5x0.75
10-34 USF
Jan - Jun/Sep 1928
Double Venturi
Inline Main & Cap Jets
No Secondary Well
Ford Drawing & S.B.
Normal Altitude
(@ 1 Meter Head Pressure)
(39.4" head pressure)
Primary
24 mm ID

0.945"

Secondary
10 mm ID

0.394"


Marked
'20'

degrees

Used with round
idle Priming Hole.


Marked '10'
3-5/64" OAL



Used with DV
lower casting only.
148 - 152

Marked '19'
(Zenith 30035A)


(0.0374")
139 - 143

Marked '18'
(Zenith 30047)


(0.0354")


Marked '19'
bottom orifice

(Zenith 30085)

(0.0365"
to 0.0380")
June - Sept 1928
Single Venturi
@ SV Introduction

Inline or Offset
Main & Cap Jets
with Secondary Well
Ford Drawing & S.B.
Normal Altitude
(@ 37-1/4" Head Pressure)
21.5 mm
27/32"
0.843"


Marked
'18-1/2'

degrees

Used with
keyhole-shaped
idle Priming Hole.
45 - 55

Marked '11'
3" OAL

(Zenith 30056A)

(0.55mm)
(0.0216" ± 0.002")
59.5mm siphon
157 - 161

Marked '19.5'
157 - 161

Marked '19'
(Zenith 30048A)

(0.95mm /
0.0374")


Marked '21'
March 1930
Single Venturi
Specs Update

Offset Main & Cap Jets
with Secondary Well
Ford Drawing & S.B.
Normal Altitude
(@ 37-1/4" Head Pressure)
21.5 mm ID
27/32"
0.843"


Marked
'18-1/2'

degrees

Used with
keyhole-shaped
idle Priming Hole.
45 - 55

Marked '11'
3" OAL

(Zenith 30056A)

(0.0216"
to 0.0236")
2-5/16" siphon
165 - 169

Marked '20'
(Zenith 30036B)

1930 #63 / #62
(1mm / 0.0394")

1931 #60
(0.0394"
to 0.0409")
157 - 161

Marked '19'
(Zenith 30048A)

(0.95mm /
0.0374")
180 - 190

Marked '20'
(Zenith 30086A)

#60
(0.0394"
to 0.0409")
Higher Altitude
Greater than 5000 ft
above sea level.

Single Venturi
with Secondary Well

Ford Drawing
(@ 37-1/4" Head Pressure)
21.5 mm
27/32"
0.843"


Marked
'18-1/2'

degrees

Used with
keyhole-shaped
idle Priming Hole.
45 - 55

Marked '11'
3" OAL

(Zenith 30056A)

(0.0216"
to 0.0236")
2-5/16" siphon
157 - 161

Marked '19.5'
157 - 161

Marked '19'
(Zenith 30048A)

(0.95mm /
0.0374")
180 - 190

Marked '20'
(Zenith 30086A)

#60
(0.0394"
to 0.0409")

Several Current Model A Recommendations to Consider

Flow Rate
Recommendations
cc's/min (ml/min)
(@ 36" Head Pressure)
Venturi
A-9586
Throttle Plate
A-9585
Idle Jet
A-9542
(manifold vacuum)
Main Jet
A-9534
(venturi suction)
Comp Jet
A-9575
(atmospheric pressure)
Cap Jet
A-9538
(venturi suction)
Cap discharges
Comp + GAV
David Renner 2022*
*ethanol fuel era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
50 - 60
target enables
greater idle air
mixture control
150
target enables
greater GAV
utilization
150
target enables
greater GAV
utilization
300
target enables
greater GAV
contribution
David Renner 2022*
Higher Elevation
*ethanol fuel era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
50 - 60
target enables
greater idle air
mixture control
125 -129
target enables
greater GAV
utilization
125 -129
target enables
greater GAV
utilization
300
target enables
greater GAV
contribution
Chris Pelikan 2010*
*ethanol fuel era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
44 - 48 150 - 160 155 - 165 170 - 190
Al Blatter 1983
NOS Jets = (full rich)
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
46 - 50
159 - 163
152 - 156
176 - 180

Carburetor and Fuel Performance Comments

It should be noted that various other hobbyist flow recommendations shown below were from a time before the widespread and predominant availability and use of E10 (typically 87, 89, 91, and 93 octane) and E15 (88 octane) ethanol-blended fuels. E15 ethanol fuel is also often labeled at pumps as "Unleaded 88", or "Regular 88". Ethanol fuel (also called oxygenated fuel) use in the U.S. has increased dramatically from about 1.7 billion gallons in 2001 to about 14 billion gallons in 2022. Per the U.S. Department of Energy, E10 is sold in every state. Unleaded 87 octane E10 is now (2023) the most commonly used fuel in the U.S.

More than 98% of U.S. gasoline contains up to 10% ethanol in order to boost octane and combustion efficiency, meet air quality requirements, or satisfy the Renewable Fuel Standard. This adoption of ethanol-blended fuels has been steadily ramping up in the U.S. since the 2001 new vehicle model year, driven by prior Energy Policy Acts and the Clean Air Act of 1990 legislation.



Ethanol contains about 33% less energy than pure gasoline, gallon for gallon. According to EPA research, modern vehicles will typically go 3% to 4% fewer miles per gallon on E10, and 4% to 5% fewer on E15 than on 100% gasoline.

Ethanol molecules contain oxygen atoms. Gasoline molecules do not. An effect of the oxygen in ethanol is that ethanol blends tend to run "leaner" in carburetors than pure gasoline because there is more oxygen available in the fuel-air mixture. Ethanol is also corrosive in nature due to the oxygen content.

Ethanol-blended fuels are also prone to phase separation in the fuel tank and carburetor bowl, and the ethanol then absorbs moisture from the air. The moisture and oxygen in the ethanol contribute to fuel system corrosion, both in the tank and in the carburetor itself, affecting iron, steel, aluminum, and zinc. The ethanol also adversely affects many older rubber compounds used in fuel systems.

For these reasons it is generally recommended to avoid ethanol fuels when possible in the Model A, and to only use non-ethanol gasoline, also commonly referred to and sold as 'recreational gas' (for boats, lawn equipment, etc.). Having stated all that, a great many Model A's are routinely run successfully on 87 octane ethanol-blended pump gas, especially if they are driven regularly.



Many jet flow recommendations shown below appear IMO to be a bit on the 'too lean' side of the equation for ethanol-blended fuels, although they may be suitable at higher elevations or for use with non-ethanol gasoline.

The specific problem with a "too lean" Idle Jet flow target is that although you can always effectively 'add more air leak' (less fuel) with the Idle Air Mixture Needle and make the mixture more lean, there is no opposite ability to 'add more fuel' to richen the idle mixture.

The only method to add any more idle fuel (needed for ethanol blends) is to enlarge the Idle Jet orifice, thus increasing its fuel flow rate and fuel consumption. Also note that the GAV in a properly restored Zenith has no effect on the idle of a warmed up engine running at normal idle speeds (<550 RPM).

Running the Main Jet flow rate a little lower than original is acceptable because you can also run the GAV a little more open in order to increase the total fuel quantity to the Cap Jet if needed. But this method only works if the Cap Jet is also sized to flow the combined Compensator + GAV values (supplied at atmospheric pressure). Many Cap Jet targets listed below may be too small to support that method of operation at higher speeds. In fact, the Cap Jet flow rates below are even less than an original Zenith carburetor, thereby possibly limiting the GAV usefulness at higher loads/speeds.

Also recognize that the flow through the Main Jet is progressively larger with increasing engine speed and venturi suction, whereas the flow through the Cap Jet is mostly constant with increasing engine speed and venturi suction.

Although the Cap Jet itself is under venturi suction effect, its direct fuel supply maximum flow rate is affected only by atmospheric pressure in the float bowl, and is unaffected by manifold vacuum or venturi effects.

The Compensator Jet flow to the Cap Jet is metered by the Compensator Jet orifice size, and by the atmospheric pressure in the float bowl on the Compensator Jet inlet side, balanced by the atmospheric pressure also on the Compensator Jet outlet side (via the vacuum breaker hole at the top of the Secondary Well).

Additionally, any fuel added to the Cap Jet supply through the GAV needle/seat bypasses the Compensator Jet orifice restriction, and is also controlled only by atmospheric pressure in the float bowl and the amount the GAV needle and seat is opened.

Other Published Model A Values

Flow Rate
Recommendations
cc's/min (ml/min)
(@ 36" Head Pressure)
Venturi
A-9586
Throttle Plate
A-9585
Idle Jet
A-9542
Main Jet
A-9534
Comp Jet
A-9575
Cap Jet
A-9538
Cap discharges
Comp + GAV
Al Blatter 1983
Recommended Targets
pre-ethanol era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
46 - 50
130 - 145
(advised 145)
152 - 156
176 - 180
Lloyd Kerr 1987
Fred Carlton
pre-ethanol era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
44 - 48 140 - 150 138 - 142 160 - 166
Steve Pargeter 2001 Ver.7
pre-ethanol era
(2015 Ver.8 is unchanged)
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
44 - 48 150 - 160 138 - 142 150 - 185
Chris Pelikan 2001
pre-ethanol era
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
44 - 48 150 - 160 138 - 142 150 - 185
Rex Reheis 2004
Gordon Biggar
(references Pargeter)
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
44 - 48 140 - 150 138 - 142 150 - 185
Paul Moller 1972-1985
(he recommended flow
testing, but gave no
flow targets)
21.5 mm
27/32"
0.843"

Marked
'18-1/2'
Your Personal
Lucky Lotto Numbers

21.5 mm
27/32"
0.843"

Marked
'18-1/2'

*Pre-ethanol era is before the widespread and predominant availability and use of E10 (87/89/91/93 octane) and E15 (88 octane) ethanol-blended fuels in the US. E15 ethanol fuel is also often labeled at pumps as "Unleaded 88", or "Regular 88".




1932-34 Model B Zenith Carburetor ~ Jet Sizes & Flow Rates

External
Adjustments
Venturi
B-9586
Power Jet
B-9594
Idle Jet
B-9542
Main Jet
B-9534
Comp Jet
B-9575
Cap Jet
B-9538
Cap discharges
Comp + GAV
Idle Air Mixture
Screw (Needle)

meters air/vacuum
in the idle mixture
circuit for closed
throttle operation.

Idle needle angle is
33 degrees included.


Throttle Lever Stop-
Screw
sets the idle
RPM, after the Idle Air
Mixture Screw (Needle)
has been tuned.


Gas Adjusting Valve
needle & seat
meters
added fuel to Cap Jet,
direct from float bowl.
GAV affects off-idle.
GAV does not affect
idle speed or quality.

GAV needle angle is
30 degrees included.

GAV full flow = 150
@ one turn open*
Single Venturi Power Jet meters
fuel from float
bowl into Power
Jet Tube Well

Fuel in the
Power Jet Well
is delivered
through the
siphon tube to
the Aperture in
the throttle bore.

Vacuum on the
Power Jet Tube
is applied by the
throttle shaft
notch design.

The Power Jet
circuit supplies
progressively
more fuel as the
throttle opens.
Supply to Idle
Jet from float
bowl is supplied
by the Idle Jet
Siphon Tube in
the Idle Jet Well.

Throttle position
applies vacuum
on Cap Jet and
diverts fuel from
the Idle Jet Well
& Idle Jet circuit
as throttle opens.

The B1 Carb uses
a brass Throttle
Plate having a
fine slot aligning
with the round
Idle Priming
Hole in the
casting bore.

The B2 carb uses
an unslotted solid
brass Throttle
Plate, together
with a slotted
brass Idle Priming
Hole fixed in
the casting bore.
Supply to Main
Jet is direct
from float bowl.

Throttle position
under load
applies vacuum
on Main Jet via
Venturi.
Comp Jet meters
fuel from float
bowl to Cap Jet
(and to the Idle
Jet Well).

Comp Jet output
is unaffected by
throttle position.
Cap Jet supply
is metered by the
Comp Jet, and is
augmented by the
GAV needle & seat.

Throttle position
off-idle applies
vacuum on Cap
Jet via Venturi.
Model B Zenith Fuel Level:
The fuel level (not the float height) in the 1932-34 Model B Zenith carburetor float bowl was originally set and regulated at 5/8" ± 1/32" below the fuel bowl gasket surface, per 1932 Ford Service Bulletins page 9, and 1934 Ford Service Bulletins page 227.

Use an external visible sight gauge mounted to the drain plug hole to measure the fuel level in the float bowl of an installed carburetor, or use a glass jar attached to the upper casting in place of the lower bowl casting. Use fuel or mineral spirits, not water, to set the fuel level (float height).

It should also be noted that the carburetor itself is horizontal in the vehicle-installed position, even though the engine is inclined at 3.25 degrees to horizontal. The intake manifold flange is angled 3.25 degrees to produce the horizontal condition when the carburetor is installed on the engine.
Model B Zenith Power Jet:
The Power Jet adds progressively more fuel as the throttle is opened, supplementing the Main and Comp jets.
This explains the lower Main + Comp value (compared to a Model A Zenith), yet for a carburetor which has a larger throttle bore diameter and a slightly larger venturi and supports higher airflow. The total fuel delivered is progressively larger as the throttle opens, along with all necessary airflow (oxygen) for that fuel.
*Note:
Per the original 1932-34 Ford Service Bulletins and Instruction Books, the Model B GAV (Gas Adjusting Valve) should only be opened a maximum of 1/4 turn for starting and warm-up, and not be operated in any open condition thereafter.

Original Model B Ford Jet Flow Rates

Flow Rate
Ford Specifications
cc's/min (ml/min)

Thread Size
Venturi
B-9586
Power Jet
B-9594

8-36 NF
Idle Jet
B-9542
Main Jet
B-9534

M5x0.75
10-34 USF
Comp Jet
B-9575

M5x0.75
10-34 USF
Cap Jet
B-9538

M5x0.75
10-34 USF
Normal Altitude
Model B w/ fuel pump
#44 float valve

Ford Drawing & S.B.
(@ 37-1/4" Head Pressure)
22 mm ID
0.866"

(Zenith 33018B)
Marked '18'
139 - 143
#65 drill
0.035 - 0.036
Marked '12'
0.0236 - 0.0241
Marked '19'
148 - 152
#63 drill
(0.0374")
Marked '18'
139 - 143
0.0354 drill
Marked '35'
#50 drill
High Altitude
Model B w/fuel pump

#44 float valve
Ford Drawing
(@ 37-1/4" Head Pressure)
22 mm
0.866"
(Zenith 33018B)
Marked '18'
139 - 143
#65 drill
0.035 - 0.036
Marked '12'
0.0236 - 0.0241
Marked '18'
131 - 135
(0.0354")
Marked '17'
120 - 124
0.0326 drill
Marked '35'
#50 drill
O-6989 Zenith for
Model A w/o fuel pump
Normal Altitude
#54 float valve

Zenith Service Manual
(@ 37-1/4" Head Pressure)
20 mm
(0.787")
Marked '13'
???
Marked '12'
0.0236 - 0.0241
Marked '18'
131 - 135
(0.0354")
Marked '17'
120 - 124
0.0326 drill
Marked '35'
#50 drill

Several Current Model B Recommendations to Consider

Flow Rate
Recommendations
cc's/min (ml/min)
(@ 36" Head Pressure)
Venturi
B-9586
Power Jet
B-9594
Idle Jet
B-9542
Main Jet
B-9534
Comp Jet
B-9575
Cap Jet
B-9538
Cap discharges
Comp + GAV
David Renner 2022*
*ethanol fuel era
22 mm
0.866"
134 - 138 50 - 60 150 (target) 150 (target) 320 - 478
Ben Nieman ~2006 22 mm
0.866"
145
0.033
60
0.022
162
0.035
135 - 145
0.033
520
Rex Reheis 2004
Gordon Biggar
22 mm
0.866"
110 - 118 ??
#65
0.035
43 - 47
#73
136 - 142
#63
0.037
110 - 118 ??
#65
0.035
372 - 462
#50
Your Personal
Lucky Lotto Numbers




Many 'Carburetor' Problems Are Actually Electrical

Electrical 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 when applying the brakes.
  2. Spark Plugs are dirty or have incorrect gap.
    Clean and set spark plug gaps to 0.035"
  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 distributor contact point stud under the top plate and the lower distributor plate and condenser 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.
  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 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. Solder the terminal end connector to each end of the coil wire.
  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. 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. Throw away your timing lights, pulley indicators, special wrenches, over-thought timing gadgets, and magic beans!


Won't Idle Properly and/or Stalls When Stopping

Fuel System:

  1. Do you have clean fuel, in a clean vented tank, and free flow out the line at the carburetor while the tank valve is open?
    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.
  3. Verify the Fuel Strainer screen (in the non-sidebowl carburetor) is not clogged with rust or debris, limiting flow volume into the float 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 position, and can be opened to the wide-open throttle position when installed on the engine and in the car.
    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.
  5. 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 to the drain plug hole, or mount a glass jar attached to the upper casting in place of the lower bowl casting. Use fuel or mineral spirits, not water, to set the fuel level (float height).
  6. Idle RPM is set too low (<350) to recover after the throttle is suddenly released/closed.
  7. 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.
  8. The Idle Air Mixture Screw (Needle) is improperly adjusted and the idle mixture is too lean (too much air/too little fuel).
  9. 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 plate screws. Re-establish the correct idle speed with the idle stop screw on the throttle shaft lever.
  10. 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.
  11. A vacuum leak to the atmosphere is present at the carburetor-to-manifold flanges or gasket.
  12. The Idle Jet orifice/flow is too small/lean, especially for ethanol mix fuels.
    Ethanol fuels have lower energy per volume compared to gasoline and need larger fuel flow rates.
  13. The Idle Jet orifice is obstructed with rust or dirt.
  14. The Idle Fuel aperture (also called the 'Priming Hole') in the upper casting throttle bore is obstructed with rust or dirt.
  15. The Idle Fuel passage in the upper casting is obstructed with rust, dirt, insect nests, or cocoons.
  16. The two small orifices in the bottom of the brass Secondary Well (0n Model A) are partially plugged with rust or dirt. This causes the idle jet supply Secondary Well to run dry.
  17. The brass Secondary Well (on Model A) is obstructing or is contacting 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 link in the list below.
  18. A vacuum leak is present at any of the various vacuum line connections of the wiper motor.
  19. A vacuum leak is present at the intake manifold-to-block gasket.


No Power at Speed and/or Speed is Limited

Fuel System:

  1. Do you have clean fuel, in a clean vented tank, and free flow out the line at the carburetor while the tank valve is open?
    Okay, let's find out why it has no power at speed, or is speed limited!
  2. First, verify the fuel line is not extended too far into the upper casting, contacting the Fuel Strainer and limiting flow volume into the carburetor.
  3. Verify the Fuel Strainer screen (in the non-sidebowl carburetor) is not clogged with rust or debris, limiting flow volume into the float 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 position, and can be opened to the wide-open throttle position when installed on the engine and in the car.
    Ensure the use of the correct length throttle control rods at the carburetor (10-5/8") and at the hand throttle (5-7/8"). Those are center-to-center dimensions.
    Loosen the clamp at the bottom of the mast jacket on a 2-Tooth steering column to allow it to rotate. This 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.
  5. 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 to the drain plug hole, or a glass jar attached to the upper casting in place of the lower bowl casting. Use fuel or mineral spirits, not water, to set the fuel level (float height).
  6. Ensure the Main Jet orifice is not obstructed with rust or dirt.
  7. Ensure the float bowl passage to the Main Jet is not obstructed with rust or dirt.
  8. Ensure the float bowl passage to the GAV is not obstructed with rust or dirt.
  9. Ensure the Cap Jet orifice is not obstructed with rust or dirt.
  10. Ensure the lower casting passage between the GAV and Secondary Well/Cap Jet is not obstructed with rust or dirt.
  11. 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).
  12. 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.
  13. Ensure 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, thus defeating the GAV function.
    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, very similar appearance).


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

Be wary of unverified advice and 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. I make a very strong effort to keep Ford Garage centered on verified facts, and I welcome 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, and from various Dykes carburetor books and publications from the 1930's, as well as from discussion and collaboration with a few knowledgeable Zenith experts, and my own extensive collection of Model A and B Zenith carburetors and Zenith rebuilding experience.

If you are looking online for expert knowledge, insight, and advice on the Model A and B Zenith, as well as quality Marvel and Model B Zenith new parts and services, new flow-tested Zenith jet sets, as well as nice design 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.

If you are looking for good Zenith rebuilding books or magazine articles, all of the 'classics' from the hobby authors of the 1960's-90's contain errors and can easily lead you astray if you accept everything at face value. A combination of Rex Reheis' (OOP) and Steve Pargeter's (Ver.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 is about the best you will be able to find in recent print for that specific carburetor.

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

The MARC/MAFCA jointly-published Restoration Guidelines & Judging Standards 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 correct and helpful and are linked in the list below.

If you are looking for helpful Zenith carburetor rebuilding advice and videos online, Richard Tompkins in Covina, California often shares knowledgable Zenith experience in Facebook group posts and on YouTube.

Tillotson? You are on your own!



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 Carburetor Float Mass
  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 Details & Masses
  10. Model B & 46 Zenith Float 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 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

May 2005