Ford Mustang/Cougar 1964-1973 Repair Guide

System Operation


The carburetor is a seemingly mystical device which is mounted on the intake manifold and performs the following tasks; opens and closes its intake port to maintain a predetermined amount of gasoline in its reservoir; channels air into the engine through the air cleaner; atomizes measured amounts of gasoline and introduces the vapor into the incoming air stream; provides metered amounts of air-fuel mixture to the deceleration valve while it is operating, is a vacuum source for the distributor vacuum advance mechanism; and, in addition, must change the air-fuel mixture it supplies to the engine for normal warm weather driving, acceleration, maximum power, cold engine operation, and idling. However, there are many carburetor adjustments and repairs that can be performed by those who have a certain amount of mechanical aptitude. Take your time, concentrate on what you are doing, and don't let it intimidate you; remember it's only a carburetor.


As previously stated, the carburetor contains a reservoir for gasoline. Two devices in the carburetor control the flow of gasoline into this reservoir: the needle valve and its seat, and the float. When fuel enters the carburetor it passes around the needle valve, through the needle valve seat, and into the reservoir. As the level of the fuel in the carburetor increases, it contacts the float, moving the float upward. A tang on the end of the float contacts the needle valve and, as the float moves upward, the tang on the float pushes the needle valve into its seat, limiting the flow of gasoline into the engine. When the float reaches a predetermined level, the float tang pushes the needle valve completely into its seat, preventing any more gasoline from entering the carburetor. As the engine uses fuel from the carburetor, the float drops down allowing more fuel to enter the carburetor. If the float is adjusted in such a way that it does not close the needle valve when the reservoir is full, or if the needle valve does not completely seal on its seat, the reservoir will overfill and dump raw gasoline into the engine. If the float is adjusted to close the needle valve too soon, the reservoir will contain an insufficient supply of gasoline, and the engine will run lean under some conditions, causing hesitation or stalling.


When the engine is operating at a constant speed above idle, the passage of air through the center of the carburetor (throttle bore) and into the engine draws fuel from the carburetor; the carburetor does not inject gasoline into the engine. This drawing of gas out of the carburetor is due to pressure, vacuum, and other such matters. The throttle plates control the flow of air through the carburetor. These are small discs mounted in the base of the carburetor. When the throttle plates are closed, no air can pass into the engine, so no gasoline can be drawn from the carburetor. The accelerator pedal of the car is connected to the throttle plates, so that when you push down on the pedal, you open the throttle plates. The greater the throttle-plate opening, the greater the amount of air that can flow through the carburetor and into the engine.

Gasoline, on its way from the carburetor reservoir to the throttle bore, passes through metering jets which determine the amount of gasoline that will enter the throttle bore. These jets are screwed into an internal part of the carburetor housing. They are not adjustable; the only way that fuel supply to the engine can be increased is by changing the jets.


2 and 4 Barrel Carburetors

Since the main metering jet determines the amount of fuel that can be drawn into the engine by the air passing through the carburetor throttle bore, the size of these jets must necessarily be a compromise. The smaller the jets, the greater fuel economy will be (to a certain end point) and the larger the jets are, the better performance will be. (Once again, this is only up to a certain point.) Therefore, a jet size is picked that will give the best all-around performance. However, this compromise jet size cannot supply a sufficient amount of fuel for wide-open throttle operation. Therefore, the carburetor contains a power valve to supplement the normal fuel supply to the engine during wide-open throttle. The power valve is normally closed but is allowed to open by low intake manifold vacuum. While it is open, additional fuel passes through the power valve circuit in the carburetor, bypassing the main metering jets, and then combining with the normal fuel supply to enrich the air-fuel mixture supplied to the engine. If the power valve becomes defective, it will open too soon, or remain partially open at all times, causing the air-fuel mixture to be too rich.


When you push down quickly on the pedal to accelerate, the throttle plates open very quickly. Thus the air flow through the carburetor increases very quickly. However, for a brief moment, this rapidly increased air flow cannot draw a proportionately increased amount of gas from the carburetor. To compensate for this, the carburetor contains an accelerator pump.

The accelerator pump is a diaphragm pump mounted on the body of the carburetor and connected, by linkage, to the carburetor throttle linkage. When the throttle linkage is moved to accelerate the car, the movement produces a corresponding movement of the accelerator pump linkage. The movement of the accelerator pump linkage compresses the accelerator pump diaphragm and the compression of the diaphragm forces a stream of gasoline into the throttle bore of the carburetor. This occurs only while the accelerator pedal is being depressed to give a greater throttle plate opening. The accelerator pump is completely mechanical in operation and not dependent on the amount of air flowing through the carburetor. The purpose of the accelerator pump is to momentarily supplement the fuel flow from the carburetor during acceleration, thereby preventing a lean air-fuel mixture. If the accelerator pump system is not working properly this lean condition will cause the engine to hesitate on acceleration. Accelerator pump problems can be caused by a defective diaphragm or spring, a defective check ball, binding linkage, or clogged passages in the system. A good way to check the operation of the accelerator pump system is to look into the top of the carburetor with the air cleaner removed and the engine not running. Have a friend depress the gas pedal. A stream of gasoline should appear in the throttle bore of the carburetor, as soon as the pedal moves, and continue until the pedal is fully depressed.


During cold engine operation, the engine requires a richer air-fuel mixture. The choke system makes this richer mixture possible. The choke system consists of a choke plate which is mounted in the carburetor air intake horn, a temperature-sensitive coil spring which is contained in the choke housing on the side of the carburetor, a choke plate vacuum pull-down device, and a choke unloader tab, and a fast idle cam.

During choke operation, the choke plate closes to limit air flow into the carburetor and intake manifold vacuum pulls a proportionately richer air-fuel mixture into the engine than during hot engine operation.

Application of the choke is controlled by the choke spring. This coil spring is connected to the choke plate. It expands when cold and contracts when hot. When the engine has not been running for a while and the ambient temperature is low, the coil spring expands and exerts pressure on the choke plate. When the accelerator pedal is depressed before starting the engine, this spring pressure forces the choke plate closed.

The fast idle cam is also attached, by linkage, to the choke plate. The fast idle cam is a stepped cam which is pulled up into its working position when the accelerator pedal is depressed before starting the engine. A fast idle speed adjusting screw, which is attached to the throttle shaft, contacts the cam and, by holding the throttle plates open, raises the idle speed of the engine to the required level.

Engine exhaust manifold heat passes through the choke coil spring housing while the engine is operating. As the engine reaches operating temperature, the increased temperature of the exhaust manifold causes the spring to contract. This removes spring pressure from the choke plate and allows the choke plate to return to the open position. This movement of the plate also disengages the fast idle cam and allows the engine idle speed to return to normal.

The choke plate vacuum pull-down device opens the choke plate slightly when the engine has started and continues to function after the choke is off. On some engines, the device consists of a piston which is contained in the choke housing. On other engines, it consists of a diaphragm and spring which are contained in an extension on the choke housing. When the engine is running, intake manifold vacuum is applied to the diaphragm or piston. The diaphragm or piston then exerts a corresponding pull on the choke plate. When the engine is cold, and the choke coil spring is exerting pressure on the choke plate, the pull of the diaphragm or piston opens the choke plate slightly. After the engine has reached normal operating temperature and the choke spring has contracted, the diaphragm or piston holds the choke plate in the full open position.

The choke unloader is a tab which is attached to the carburetor throttle shaft, opposite the fast idle adjusting screw. When the accelerator pedal is depressed to the full open position, the fast idle adjusting screw moves away from the fast idle cam and the unloader tab moves up to contact the cam. The cam, through its connecting linkage, then partially opens the choke plate. This allows additional air to enter the engine, and serves to aid in starting a flooded engine.