Chevrolet Citation/Omega/Phoenix/Skylark 1980-1985 Repair Guide

Introduction

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No idle speed or mixture adjustments are possible on 1982 and later fuel injected engines.

This section contains only carburetor adjustments as they normally apply to engine tune-ups. Descriptions of the carburetors and complete adjustment procedures can be found in Section Four.

When the engine in your car is running, air/fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the downward movement of the pistons on the intake stroke of the fourstroke cycle of the engine. The amount of air/fuel mixture that enters the engine is controlled by throttle plates in the bottom of the carburetor. When the engine is not running, the throttle plates are closed, completely blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected, through the throttle linkage, to the gas pedal. What you are actually doing when you depress the gas pedal is opening up the throttle plates in the carburetor to admit more of the fuel/air mixture to the engine. The further 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 remain closed. When the engine is idling, it is necessary to open the throttle plates slightly. To prevent having to keep your foot on the gas pedal when the engine is idling, an idle speed adjusting screw was added to the carburetor. This screw has the same effect as keeping your foot slightly depressed on the gas pedal. The idle speed adjusting screw contacts a solenoid on the outside of the carburetor. When the screw is turned in, it opens the throttle plates on the carburetor, raising the idle speed of the engine. This screw is called the curb idle adjusting screw and the procedures in this section will tell you how to adjust it.

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 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 adjusting screw which restricts the amount of air/fuel mixture that enters the engine at idle.

On the X-Body cars, the idle mixture screws are concealed under staked-in plugs. Idle mixture is not considered to be a normal tune-up procedure, because of the sensitivity of emission control adjustments. Mixture adjustment requires not only special tools with which to remove the concealing plugs, but also the addition of an artificial enrichment substance (propane) which must be introduced into the carburetor by means of a finely calibrated metering valve. These tools are not generally available, and require a certain amount of expertise to use. Therefore, mixture adjustments are purposely not covered in this guide. If you suspect that your car's carburetor requires a mixture adjustment, it is strongly recommended that the job be referred to your dealer or a qualified mechanic with specific training in making the adjustment.

 
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