GM Full-Size Trucks 1970-1979 Repair Guide

Ignition Timing



See Figure 1

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Fig. Fig. 1: The timing marks are located on the vibration damper on this 1977 V8 engine

Timing should be checked at each tune-up and any time the points are adjusted or replaced. It isn't likely to change much with HEI. The timing marks consist of a notch on the rim of the crankshaft pulley or vibration damper and a graduated scale attached to the engine front (timing) cover. A stroboscopic flash (dynamic) timing light must be used, as a static light is too inaccurate for emission controlled engines.

There are three basic types of timing light available. The first is a simple neon bulb with two wire connections. One wire connects to the spark plug terminal and the other plugs into the end of the spark plug wire for the No. 1 cylinder, thus connecting the light in series with the spark plug. This type of light is pretty dim and must be held very closely to the timing marks to be seen. Sometimes a dark corner has to be sought out to see the flash at all. This type of light is very inexpensive. The second type operates from the vehicle battery; two alligator clips connect to the battery terminals, while an adapter enables a third clip to be connected to the No. 1 spark plug and wire. This type is a bit more expensive, but it provides a nice bright flash that you can see even in bright sunlight. It is the type most often seen in professional shops. The third type replaces the battery power source with 110 volt current.

Some timing lights have other features built into them, such as dwell meters or tachometers. These are convenient, in that they reduce the tangle of wires under the hood when you're working, but may duplicate the functions of tools you already have. One worthwhile feature, which is becoming more of a necessity with higher voltage ignition systems, is an inductive pickup. The inductive pickup clamps around the No. 1 spark plug wire, sensing the surges of high voltage electricity as they are sent to the plug. The advantage is that no mechanical connection is inserted between the wire and the plug, which eliminates false signals to the timing light. A timing light with an inductive pickup should be used on HEI systems.


See Figure 2

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Fig. Fig. 2: View of the timing marks (on the pulley) and timing indicator - 1975-78 gasoline V8 engines

  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. You can also use the No. 6 wire, if it is more convenient. 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 any, to get the speed down. Otherwise, adjust the idle speed screw. This is done to prevent any centrifugal (mechanical) advance.

The tachometer hookup for 1970-74 models is the same as the dwell meter hookup shown in the Tune-Up and Troubleshooting sections. On 1975-77 HEI systems, the tachometer connects to the TACH terminal on the distributor (V8) or on the coil (6-cylinder) and to a ground. For 1978-79 vehicles, 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 with HEI.

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.