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Checking & Adjustment GM Chevy Mid-Size Cars 1964-1988 Repair Guide

See Figures 1, 2 and 3

The vehicle emission label, which is found underhood, will often contain specifications or procedures for checking and adjusting timing that have been updated during production. The information contained on the label should always be used if it differs from these instructions.

Fig. 1: Connect the timing light to the No. 1 spark plug wire. This type using an inductive clamp is easiest to use

The tachometer hookup for cars 1964-74 is the same as that shown for the dwell meter in the Tune-Up section. On 1975-77 HEI systems, the tachometer connects to the TACH terminal on the distributor for V8s, or on the coil for 6-cylinders, 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.

1. Set the parking brake and block the drive wheels, warm the engine to normal operating temperature. Shut off the engine and connect the timing light to the No. 1 spark plug (left front on V8 and V6 or front on an inline 6-cyl.). Do not, under any circumstances, pierce a wire to hook up a light.

Fig. 2: Common timing mark scale found on the front of the engine

Fig. 3: With the engine running, aim the timing light at the marks to determine ignition timing

2. Clean off the timing marks, then label the pulley or damper notch and the timing scale with while chalk or paint for better visibility. If the timing notch on the damper or pulley is not visible from the top, the crankshaft should be bumped around using the starter or turned using a wrench on the front pulley bolt, in order to bring the mark to an accessible position.

3. Disconnect and plug the vacuum advance hose (if equipped) at the distributor, to prevent any distributor advance. The vacuum line is the rubber hose connected to the metal cone-shaped canister on the side of the distributor. A short screw, pencil, or a golf tee can be used to plug the hose.

1981 models with Electronic Spark Timing have no vacuum advance, therefore you may skip the previous step, but you must disconnect the four terminal EST connector in order to disable the electronic spark advance before proceeding.

4. Start the engine and adjust the idle speed to specification, refer to the Tune-Up Specifications chart. Some cars require that the timing be set with the transmission in Neutral. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw. This is to prevent any centrifugal advance of timing in the distributor.

		WARNING Never ground the HEI TACH terminal; serious system damage will result, including ignition module burnout.

5. Aim the timing light at the timing marks. Be careful not to touch the fan, which may appear to be standing still. Keep your clothes and hair, and the light's 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° mark on the scale. Either B, BTDC, or Before Top Dead Center, may be shown as BEFORE on the scale, while A, ATDC or After Top Dead Center, may be shown as AFTER.

6. Loosen the distributor base clamp locknut. You can buy special wrenches which make this task a lot easier on V8 and V6 engines. 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 to advance.

The 231 V6 engine has two timing marks on the crankshaft pulley. One timing mark is ¹/₈ in. wide and the other, four inches away, is ¹/₁₆ in. wide. The smaller mark is used for setting the timing with a hand-held timing light. The larger mark is used with the magnetic probe and is only of use to a dealer or garage that is equipped with such equipment. Make sure you set the timing using the smaller mark.

7. Once the timing is properly set, hold the distributor to keep it from the turning and tighten the locknut. Check the timing again after finishing with the nut in case the distributor moved as you tightened it.

8. If applicable, remove the plug and connect the distributor vacuum hose.

9. If necessary check and/or adjust the idle speed.

10. Shut off the engine and reconnect the EST wire (if equipped), then disconnect the timing light and tachometer.