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Tech Q&A - Engine - Fuel System

ACCELERATION

Question:
Engine will not accelerate or go over 35 MPH.

Answer:
You may have a clogged or restricted fuel line. You are not getting enough fuel to the engine.

To check the fuel line, turn the gas shut off valve OFF and then disconnect the fuel line from the carburetor. Open the shut-off valve. There should be a good steady stream of fuel from the line. If not, remove the line to clear any blockage. The blockage may be from rust or foreign matter in the tank. You may have to drain the tank and remove the shut-off valve and inside fuel line to clear all matter. Try blowing through the line before disconnecting everything. You may also have clogged jets in the carburetor. It may need to be cleaned. Also, If you have a cast iron type sediment bowl on the firewall, it may need to be cleaned. A small screen filter can be inserted into the top of the fuel shut off valve (available from most Model A parts suppliers) that will screen out most sediment from the gas tank.

Turn the choke rod clockwise until it seats, then back it off 1/4 turn. This is the normal fuel mixture position.

This can also be caused by incorrect timing (to retarded) and to narrow gap set on points. Reset timing.
-- Les Andrews, 1998 Technical Director

GAS TANK REMOVAL

Question:
We are preparing to remove the fuel tank on our 31 Model A and have it cleaned. What is necessary in removing tank? The engine, transmission and floor board are already out of the car. Our books and manuals do not seem to answer this question for us. Also, is it recommended to have a coating on the inside of the tank? The tank does not leak but does have the usual rust inside. If a coating is put in tank, what kind and how is it applied?

Answer:
To remove the gas tank, remove the instrument panel along with the speedometer cable and the harness wires that go through the trough on the bottom of the tank. Then remove the 7 screws that attach the dash rail and remove the dash. Then remove the windshield (8 screws in the header under the top hinge) and windshield post brackets (2 screws on each side), remove the steering column support bracket, the 2 inside cardboard cowl panels, and remove 2 'C' clamps that hold the bottom flange of the tank to the side cowl panel.

Then remove the 12 bolts along the top flange (behind the dash panel), remove the front cowl band, attached to the body at both ends . Remove the 10 pan head screws and lock washers around the front edge of the gas tank. Tip the tank slightly up at the front edge and lift it out from under the dash rail support. Remove the gas shutoff valve and the gas gage. Have a Radiator shop Hot Dip it. Then coat the inside with a gas tank sealer. Major Model A parts suppliers have it. You pour it into the tank and roll the tank around to coat all surfaces. This procedure is clearly illustrated and described in the "Model A Ford Mechanics Handbook" . This is the only source I know of that describes removal of both the 28/29 style and 30/31 style gas tank.
-- Les Andrews, 1998 Technical Director

IDLE AIR ADJUST

Question:
When you turn the carburetor air adjustment screw in (clockwise), are you making the mixture richer or leaner? My 'A' has a tendency to want to stall when you slow down to turn a corner and have the clutch in to shift into second gear. I have set the idle up higher than it should be to avoid the stall problem. Does this seem like an idle mixture that is too lean or too rich? I have heard both opinions expressed.

Answer:
The carburetor air adjustment affects idle air mixture only. For this adjustment to work correctly you must have no air leaks around the manifold gasket (intake side), at the carburetor or vacuum line. Many times the throttle shaft in the upper housing of the carburetor is worn and allows air to be drawn in around the shaft.

To make correct adjustment of idle air adjustment screw begin by turning the idle air screw all the way in (clockwise) to the seat (lightly) and then back out (counter-clockwise) 1-1/2 turns. With the spark lever in the full retard position, bring the engine to an idle. Lean the carburetor mixture a little by adjusting the dash adjusting knob for no more than 1/4 turn open. Now turn the idle air screw in slowly until the engine starts to stall (Note the position of the screw). It should cause the engine to start to stall out about 1/4 turn in (counter-clockwise) from where you started. Now slowly turn the screw counter-clockwise until the engine starts to stall again. Then turn back counter-clockwise to a point in between the two stall points. This should be about 1-1/2 turns from the seat.

Turning the screw clockwise reduces the air mixture to the idle fuel, making it a richer mixture (less air). Turning the screw counter-clockwise increases the air mixture, there by providing a leaner fuel mixture. If the screw has little or no effect it means you either have an air leak in the idle circuit (usually caused by too much wear around the throttle shaft) or if you have a windshield wiper vacuum line you may have a leak in that line. Put a 1/8" brass pipe plug in the intake manifold vacuum connection to plug it off for testing (1930/1931 only with vacuum wipers).

The problem you describe can also be caused by float level set too high. When the float level is too high the fuel level in the jets is too high and causes a rich mixture when engine vacuum is reduced (as when slowing down to turn a corner). The rich mixture floods the carburetor, causing it stall or quit at reduced speeds.
-- Les Andrews, 1998 Technical Director

LEAKS

Problem:
Carburetor leaks fuel out through the air intake.

Solution:
There are three main causes of fuel leaking out the air intake.

-- Les Andrews, Technical Director

ZENITH/HOLLEY VS. TILLOTSON

Question:
I am having difficulty in understanding why in parts catalogs, the two brands of Model A carburetors are Zenith and Tillotson. But, when I look at your restoration guides (MAFCA-RESTORER) all I find are references to Zenith and Holley. What gives?

Answer:
The Model A (all years) came from the factory with a Zenith carburetor. The Zenith 1 carburetor was made by Zenith, Zenith 2 made by Holley, and Zenith 3 made by Ford. All three were essentially the same carburetor (Cast iron bowl) and all three identified as a Zenith Carburetor, although made by three different manufacturers. Later Tillotson made an after-market replacement carburetor for the Model A (Cast aluminum/pot metal), which is a different design and usually sold through Western Auto stores. The Model A Restoration Guidelines and Judging Standards only addresses original equipment.
-- Les Andrews, Technical Director

Last Updated: 11/18/23
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