Servicing Model A Carburetor Adjustment

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 .025”.
  • Breaker contact points burnt or pitted—Dress points down with an oil stone and set gap at .015” to .018”.
  • 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. 420. 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. 420. 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. 420.

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

A plugged compensator, “F,” fig. 421 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.421, 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.

Identification of Carburetor Parts

((Fig 422))

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

FIG. 423—Secondary Venturi

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

((Fig 424))

Fig 424 — Main Jet

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

Fig 425 — compensator

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

Fig 426 — Cap Jet

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

Fig 427 — 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.

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