Model a carburetor adjustment

Model a carburetor adjustment DEFAULT

The Place for Model-A Ford Carburetors

There are two external adjustments to be made on the carburetor when it is installed.

The Place for Model-A Ford Carburetors

Carburetor Idle RPM and Idle Air Mixture Adjustment

Driver's Side Engine Compartment, with FlagsZenith, Model A Ford Carburetor, with Flags for Idle RPM and Idle Air Mixture Adjustments.
  1. There are two external carburetor adjustments, and they affect only the idle performance. They are the idle RPM and idle air mixture adjustments
  2. Turn Gas Valve under dash on passenger side to open (Pointing down)
  3. Preparation: Fully warm the engine up, move spark control (on left side of steering wheel) up, turn the under dash choke control knob full clockwise and then counter clockwise about one turn, and move the hand throttle lever (on the right side of the steering wheel) all the way up. If engine tend to die, pull the throttle lever down a little.
  4. Preliminary Idle RPM adjustment: Turn the adjusting screw on the carburetor throttle arm until the end of the threads is making contact with the stop. You should now be able to put the hand throttle lever all the way up without the engine tending to die. Go back to the adjusting screw on the carburetor throttle arm and adjust it to a low RPM, just faster than where engine tends to stall. This will ensure that the carburetor idle circuit if functioning.
  5. Idle Air mixture adjustment: Slowly turn the idle air mixture screw clockwise until the engine begins to stall, and note the position. Slowly turn the idle air mixture screw counter clockwise until the engine begins to stall and note the position. Now, adjust the screw about half way between the two positions, and you will often hear a very slight increase in engine RPM. (Idle air mixture screw should be about 1–1/2 turns out from full clockwise.)
  6. Final Idle RPM adjustment: Pull the spark lever about 3/4 down. Readjust the idle RPM adjustment for the idle speed that you prefer. (If adjusted too high, you will likely tend to grind gears when driving.)

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Location: Rocklin, CA


DefaultRe: What does the Dash Carburetor adjustment adjust?

This is a draft of an article that will be in the Spring Model A Times, might help explain it to you.
Mixture Mastery
Unlike an EFI system, a carburetor is a purely mechanical device that meters air and fuel in the proportion needed for combustion. While this might seem simple enough, a carb takes advantage of a handy characteristic of nature identified way back in by the physicist Daniel Bernoulli called the Bernoulli Principle, better known as the Venturi Effect. The key aspect of the Bernoulli Principle is the inverse relationship between velocity and pressure, which dictates that as the air speed increase through a carburetor’s venture, the pressure will drop. This lower pressure is just what is needed to pull fuel out of the carb’s bowl, which is at atmospheric pressure. The beauty of a carb is that its operation is based on the simple properties of physics. Harnessing the velocity-dependent pressure drop to meter air and fuel in exactly the needed proportions over a wide range of operating requirements relies on a complex system of calibrated orifices, and those are the items that comprise what we refer to as the car’s calibration, or “tune.” What is actually being calibrated here is how the carburetor as a metering device will respond to changing pressure differentials in its various circuits.
The level of carburetor tuning that an enthusiast can practically perform will depend upon the design of the carb, and the tuning components available. Many of you run the original updraft Zenith design carb with limited adjustments. This carb needs careful rebuild requirements, to assure that correct sized jets are installed. If the jets are incorrect getting a good running adjustment is difficult if not impossible. As technology advanced down draft carburetors were placed on the Model A, one of the more popular ones was the Stromberg, which is seen quite often on cars today. One of the most common carburetors on touring Model As is the Weber downdraft. This is a modern carburetor that is sold by most of the parts houses. The most basic level here is a backyard mechanic armed with nothing more than a screwdriver a good ear to attack his carb. With a little talent you can adjust the idle mixture and speed, and call it a day. This idle adjustment screw is on the top of the carb on the Zenith/Holley carbs. There is also the idle adjustment on the throttle linkage. When adjusting this make sure that the throttle lever is fully retarded. The fuel mixture is adjusted by turning the choke knob. This adjustment can be made while driving and may be necessary when changes in driving conditions happen (altitude change, steep climbing, etc.). Again care in the rebuilding of these carburetors comes into play. If the seat for the adjustment needle is to close on are rusted and pitted no adjustment will be adequate to compensate for the fuel seeping past the damaged seat.
The use of the original carburetor requires careful evaluation if you plan on doing longer tours. There are several “new” carbs being offered that have the appearance of the original but offer new surfaces for all the seats etc. Again these are offered through various part suppliers. While we use the Weber Carburetor on the Sedan we use on the longer tours, we have taken many extended tours with the original carburetor and had great success. We will be installing the original double venture Holley carburetor on the Phaeton Project Build.
There are several books on rebuilding the original carburetors; we referred to “The Model A Ford Carburetors” by Paul Moller that was written in the early 70s. In addition we referred to Holley’s website for insight into carburetor adjustments.

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Suction created by the pumping of the pistons, causes fuel and air to flow through the carburetor into the engine. Each Alternate downward stroke draws in a fresh charge of mixed fuel and air.

The throttle plate manages the amount of air flow that is delivered to the engine and is controlled by the "Throttle Lever" and "Accelerator" inside the cab.




Pulling on the Carburetor Adjustment Knob, often called the GAV (Gas Air Valve) inside the cab opens and closes the Choke Plate. Turning the GAV enriches or leans the fuel mixture flowing through the "Cap Jet".

When the engine is started with the choke closed a greater vacuum is formed, pulling in a larger amount of fuel.






Fuel Flow

GREEN: Adjustable fuel supply for Cap Jet

BLUE: Predetermined fuel supply by Compensator Jet for Cap Jet and Idle Jet

RED: Direct fuel supply for Main Jet (Used for High Speeds)

YELLOW: Ambient air to fuel bowl




The &#;Compensating Jet&#; is inside the fuel bowl and empties into the &#;Secondary Well&#; which is open to the air. The &#;Cap Jet&#; connects with the &#;Secondary Well&#;. The &#;Cap Jet&#; can only draw as much fuel as the &#;Compensating Jet&#; will allow, regardless of the amount of suction.


The "Cap Jet" has an additional fuel supply from the "Fuel Bowl". The flow rate of the fuel to the "Cap Jet" is controlled inside the cab with the "GAV" adjustment knob. Turning it left (towards the driver) enriches the fuel mixture for the "Cap Jet". The "Cap Jet" is used at low speeds.


The &#;Main Jet&#; is connected directly to the "Fuel Bowl". It acts like a straw; the stronger the suction the greater amount of fuel will be supplied. The "Main Jet" kicks in and helps out the "Cap Jet" at higher speeds. When cruising, the "Cap Jet" can be leaned out (turn right) to conserve on fuel.

Idling occurs when the "Throttle Plate" is partially open. A vacuum above the plate is created, drawing air through a small hole, which pulls the gas from the secondary well through the "Idle Jet".

Model A Garage, Inc.

In cases of suspected carburetor trouble or complaints of poor fuel economy, first check spark plugs, breaker points, compression, etc., before removing carburetor. Many so called carburetor troubles can be traced to one or more of the following causes:

  • Dirty spark plugs; points incorrectly spaced—Clean points and set gaps to ”.
  • Breaker contact points burnt or pitted—Dress points down with an oil stone and set gap at ” to ”.
  • Leaky manifold or carburetor connections—With engine idling slowly, flow a little oil on each joint. If engine picks up speed there is a leak.
  • Poor compression—check compression in each cylinder by turning engine over slowly with hand crank.
  • Brakes dragging — Jack up car and see that all wheels revolve freely.
  • Tires soft—Inflate all tires to 35lbs. Pressure.

If the above points are OK and there is a free flow of fuel through the line, check the carburetor.

Cleaning the Carburetor

Remove filter screen. Blow out any dirt with air or rinse screen thoroughly in gasoline. The screen is easily removed by backing out the filter plug. See “A,” Fig. Usually cleaning the screen is sufficient to overcome the trouble.

For complete cleaning, remove carburetor and disassemble it by removing main assembly bolt “B.” See Fig. Separate the parts carefully to avoid damaging the gasket, float, and idling jet tube.

Remove brass plug “C” beneath main jet, and rinse carburetor bowl in gasoline or use air to blow out any dirt which may have lodged in the bottom of the bowl or in jets.

Trouble Shooting Hints

Make certain there is gasoline in the tank and a free flow of fuel through the line.

See that the secondary Venturi is right side up as shown at “D,” Fig.

On complaint of lack of speed, see that the main jet “E” is free from dirt.

A plugged compensator, “F,” fig. will result in poor idling and low speed performance.

The idling, consequently the tube and metering hole must be kept clear.

In case of leaks see that all connections and jets are tight. If damage, replace float or fuel valve assembly.

On complaint of poor fuel economy, make certain owner understands proper operation of dash adjustment.

Water in the fuel line may freeze in cold weather and stop the flow of fuel—use hot cloths for thawing.

The carburetor is a delicate instrument and should be handled carefully. Don&#;t use strong­ arm methods in taking it apart, reassembling or handling the various parts. With reason­ able care the carburetor will last indefinitely.


Do not expect a new engine that is too stiff to &#;rock&#; on compression when stopped, to idle well at low speed.

To Adjust the Idle—If engine is free, fully retard spark lever. Adjust throttle plate ad­justing screw. See&#;H,&#; Fig, so that en­gine will run sufficiently fast to keep from stalling. Turn idle adjusting screw &#;I&#; in or out until engine runs evenly without &#;rolling or skipping,&#; then back off throttle plate ad­justing screw until desired engine speed is ob­tained. (Make adjustments with engine warm.)

Usually best idling will be obtained with the adjusting screw approximately two turns off its seat.

Dash Adjustment— The dash adjustment does not control the entire fuel supply. A minimum amount of fuel is constantly drawn from the float chamber through small fixed openings even when the dash adjustment is fully closed.

For best operation under usual driving conditions, the dash adjustment should be backed one ­quarter turn off its seat. Running with the adjustment more than one­ quarter turn off its seat may be necessary on new stiff engines, but otherwise this will result in poor economy, carbon and crankcase dilution.

The dash adjustment may be turned less than one quarter turn off its seat to obtain a lean mixture suitable for high altitudes, high test fuels, or when driving at steady speeds on level roads. Under normal conditions, how­ ever, too lean a mixture causes uneven run­ning at low speeds and slow pickup.

Do not force the adjusting needle down on its seat as this will score the parts.

Cold Engine Starting

First: Open hand throttle lever two or three notches. Fully retard spark lever. Turn car­buretor dash adjustment one full turn to left.

Second: Turn on ignition. Pull back choke rod at the same time depress starter switch. The instant the engine starts, release choke.

Third: As motor warms up, gradually turn dash adjustment to the right until it is in its normal running position—one­ quarter turn off seat when engine is warm.

Starting in Cold Weather

These instructions are to aid starting at low temperatures, especially when battery efficiency is low and the engine does not turn over at starting speed.

First: Open throttle lever two or three notches. Fully retard spark lever. Open dash adjustment one full turn and crank engine two or three times with ignition off and choke pulled all the way back. This will fill the cylinders with a rich mixture.

Second: Release choke and turn on igni­tion. Engine should start on second or third quarter turn of the crank.

Warm Engine Starting

With spark control lever about half way down quadrant and throttle lever advanced two or three notches, turn on ignition and depress starter switch. It is usually unneces­sary to use choker when the engine is warm.

((Fig ))

The venturi measures the air through the carburetor and keeps it moving fast enough at low speed to completely atomize the fuel.

FIG. —Secondary Venturi

This is an auxiliary air metering tube which increases the air velocity at the jets to give quick response on acceleration.

((Fig ))

Fig — Main Jet

This is the long jet. It is connected with the fuel chamber. Its effect is most noticeable at high speeds.

Fig — compensator

The fuel in the bowl flows through this jet into the compensating well. The jet is most effective at low speeds.

Fig — Cap Jet

The cap jet controls the rate of discharge from the compensator well into the air stream.

Fig — Idling Jet

The function of the idling jet is to measure fuel for very slow running. When the throttle is open, the idling jet is put out of action as the flow of the fuel then changes direction and passes through the cap jet.


Carburetor adjustment a model

Paradise Valley Model A Ford Club

Setting up the Carburetor

Postby Late '31 » Sun Dec 18, am

Setting up the Carburetor on a Ford Model A

copyright August 8, E. Kirk Ellis Pikeville NC

The carburetor should be adjusted the way it will be operated normally. If it has an air filter, adjust it with the filter in place. If you have poor results setting the idle, the carburetor may not work properly with a filter, so try it again if necessary without the filter.

1. Start and run the car to warm it up to normal operating temperature.

2. Push the spark advance all the way up (left hand lever). The spark should be left in this position, (fully retarded), for the rest of these steps.

3. Pull the throttle down to a fast idle (right hand lever).

4. Now we will find the “sweet spot” to keep the Gas Adjusting Valve, (GAV), set at. This is the choke knob under the passenger side of the dash.

5. Open GAV 1 turn (CCW).

6. Now slowly fully close GAV, and note that the car should start idling poorly as you approach full closed (CW). Slowly open the GAV back to the point where it speeds up slightly and starts running smoothly. This will be the ideal place to set the rest of the carburetor up. It will also be about the best place for driving, though it can vary with temperature, gasoline brand, etc.

7. Push the throttle back up to an engine RPM for smooth steady idling, at a speed you prefer.

8. Adjust the idle screw so that it hits the stop on the carburetor.

9. Push the throttle all the way up. The idle screw will hold the idle at the speed you preferred, as set in step 7.

Adjust the Air Idle screw as follows. Turn it in (CW, which is richer) until the engine starts to run poorly. Note the position.

Now adjust the Air Idle screw CCW (leaner) until the engine starts to run smooth. Continue turning the Air Idle screw out CCW until the engine again starts to run poorly. Note the position.

Turn the Air Idle screw back CW until it’s about half way between the positions found in steps 10

Leave it there. You should try to find the spot where it runs smoothest between rich (CW) and lean (CCW).

Test drive the car. Check for proper performance.
Late '31
Posts: 93
Joined: Thu Dec 08, am
Pilot Air/Fuel Screw Adjustment Explained - Single Carb - Part 1

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