GM Full-Size Trucks 1980-1987 Repair Guide

Inspection and Adjustment


To check and adjust the timing:

  1. Warm up the engine to normal operating temperature. Stop the engine and connect the timing light to the No. 1 (left front on V8, front on six) spark plug wire. Numbering is illustrated in this Section. Under no circumstances should the spark plug wire be pierced to hook up a timing light. Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk. The timing notch on the pulley or damper can be elusive. The best way to get it to an accessible position for marking is to bump the engine around using either the ignition key or a remote starter.
  3. Disconnect and plug the vacuum line at the distributor. This is done to prevent any distributor vacuum advance. A short screw, pencil, or a golf tee can be used to plug the line.
  5. Start the engine and adjust the idle speed to that specified in the Tune-Up Specifications chart. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. However, it is safer to adjust the timing in Park and to reset the idle speed after all timing work is done. Some trucks require that the timing be set with the transmission in Neutral. Refer to the Tune-Up Specifications chart or the underhood sticker for details. You can disconnect the idle solenoid, if equipped, to get the speed down. Otherwise, adjust the idle speed screw. This is done to prevent any centrifugal (mechanical) advance.

Never ground the HEI TACH terminal; serious system damage will result.

  1. Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. Keep the timing light wires clear of the fan, belts, and pulleys. If the pulley or damper notch isn't aligned with the proper timing mark (see the Tune-Up Specifications chart), the timing will have to be adjusted.

TDC or Top Dead Center corresponds to 0 degrees. B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.

  1. Loosen the distributor base clamp locknut. You can buy a special wrench which makes this task a lot easier on V8s. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation (found in the Firing Order illustration in this Section) to retard, and against the direction of rotation to advance.
  3. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tighten it.
  5. Replace the distributor vacuum line. Correct the idle speed.
  7. Stop the engine and disconnect the timing light.