To check idle speed, a tachometer is used. On 1975-77 HEI systems, the tachometer connects to the TACH terminal on the distributor and to a ground. For 1978 and later models, all tachometer connections are to the TACH terminal. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won't work at all with HEI. Consult the tachometer manufacturer if the instructions supplied with the unit do not give the proper connection.
Idle mixture and idle speed adjustments are critical aspects of engine tune-up and exhaust emission control. It is important that all tune-up instructions be carefully followed to ensure good engine performance and minimum exhaust pollution. Through the succeeding model years covered in this guide, the different combinations of emissions systems on different engine models have resulted in a wide variety of tune-up specifications. See the Tune-Up Specifications chart at the beginning of this information. All models covered here have an emissions information sticker placed within easy sight in the engine compartment, giving timing, carburetor adjustment and other important tune-up information. If there is any difference between the specifications listed in this guide and those on your vehicle's emissions sticker, always follow the specs on the sticker.
The following carburetor adjustment procedures are listed by year, carburetor type, and engine displacement and code where necessary (consult the Engine Identification Code chart in this guide), as many of the carburetors covered are used simultaneously by all four GM divisions. Other carburetor adjustments and maintenance are found in Fuel System .
Idle mixture screws have been preset and capped at the factory. The caps should be removed only in the case of major carburetor overhaul, or if all other possible causes of poor idle have been thoroughly checked. If you must adjust the idle mixture, have the carbon monoxide (CO) concentration checked at a professional shop equipped with a CO meter. Mixture adjustments are included only where it is possible for the owner/mechanic to perform them.