Ford Full-Size Cars 1968-1988 Repair Guide



Always refer to the Vehicle Emission Control Information (VECI) label to verify the adjustment procedure and any other special instructions.Always follow the VECI label instructions if they differ from the service procedures.

This section contains only carburetor adjustments as they normally apply to engine tune-up. Descriptions of the carburetor and complete adjustment procedures can be found in Chapter 5 .

When the engine in your car is running, air/fuel mixture from the carburetor is drawn into the engine by a partial vacuum. The vacuum is created by the downward movement of the pistons on the intake stroke of the 4-stroke cycle of the engine. The amount of air/fuel mixture which enters the engine is controlled by the throttle plate(s) in the bottom of the carburetor. When the engine is not running the throttle plate(s) is (are) closed blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected, through the throttle linkage, to the accelerator in the passenger compartment of the car. After you start the engine and put the transmission in gear, you depress the accelerator to start the car moving. What you actually are doing when you depress the accelerator is opening the throttle plate(s) in the carburetor to admit more of the air/fuel mixture to the engine. The farther you open the throttle plates in the carburetor, the higher the engine speed becomes.

As previously stated, when the engine is not running, the throttle plates in the carburetor are closed. When the engine is idling, it is necessary to open the throttle plate slightly. To prevent having to keep your foot on the accelerator when the engine is idling, an idle speed adjustment screw is incorporated into the carburetor. The idle speed adjusting screw contacts a lever (the throttle lever) on the outside of the carburetor, and when turned clockwise, the throttle plate on the carburetor is opened, raising the idle speed of the engine.

In addition to the idle adjusting screw, most engines have a throttle solenoid positioner. Ford has found it necessary to raise the idle speed on the these engines to obtain a smooth engine idle. When the key is turned OFF , the current to the spark plugs is cut off and the engine normally stops running. However, if an engine has a high operating temperature and a high idle speed, it is possible for the temperature of the cylinder, instead of the spark plug, to ignite the air/fuel mixture. When this happens, the engine continues to run after the key is turned OFF . To solve this problem, a throttle solenoid was added to the carburetor. The solenoid is a cylinder with an adjustable plunger and an electrical lead. When the ignition key is turned to ON , the solenoid plunger extends to contact the carburetor throttle lever and raise the idle speed of the engine. when the ignition key is turned OFF , the solenoid is de-energized and the solenoid plunger falls back from the throttle lever. This allows the throttle lever to fall back and rest on the curb idle adjusting screw. which closes the throttle plates far enough so that the engine will not run on.

Since it is difficult for the engine to draw the air/fuel mixture from the carburetor with the small amount of throttle plate opening that is present when the engine is idling, an idle mixture passage is provided in the carburetor. This passage delivers an air/fuel mixture to the engine from a hole which is located in the bottom of the carburetor below the throttle plates. This idle mixture passage contains an adjustment screw which restricts the amount of air/fuel entering the engine at idle. The procedures given in this section will tell how to set the idle mixture adjusting screw(s).

When the electric solenoid is disengaged, the carburetor idle speed adjusting screw must make contact with the throttle lever to prevent the throttle plates from jamming in the throttle bore when the engine is turned OFF.