Chevrolet Full-size Cars 1968-1978 Repair Guide

Ignition Timing

Print

Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, the distributor has two means to advance the timing of the spark as the engine speed increases. This is accomplished by centrifugal weights within the distributor, and a vacuum diaphragm mounted on the side of the distributor (non HEI EST distributors only).

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

Timing marks consist of a notch on the rim of the crankshaft pulley and a scale of degrees attached to the front of the engine. The notch corresponds to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, which is hooked into the circuit of the No. 1 cylinder spark plug. Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks, the exact position of the piston within the cylinder can be read, since the stroboscopic flash makes the mark on the pulley appear to be standing still. Proper timing is indicated when the notch is aligned with the correct number on the scale.

There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light operated from the vehicle's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.

If your car has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 spark plug wire, eliminating the adapter. It is not susceptible to cross firing or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.

INSPECTION & ADJUSTMENT


Click image to see an enlarged view

Fig. Fig. 1 Example of a typical timing mark tab-it's always a good idea to clean off this tab prior to checking and adjusting the ignition timing

  1. 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 or front on an inline 6-cylinder). Do not, under any circumstances, pierce a wire to hook up a light.
  2.  
  3. Clean off the timing marks and mark the pulley or damper notch and the timing scale with white chalk or paint. The timing notch on the damper or pulley can be elusive. Bump the engine around with the starter or turn the crankshaft with a wrench on the front pulley bolt to get it to an accessible position.
  4.  
  5. Disconnect and plug the vacuum advance hose 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.
  6.  
  7. Start the engine and adjust the idle speed to the specified rpm in 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.
  8.  

The tachometer hookup for 1968-74 cars is the same as that shown for the dwell meter in the Tune-Up information. On 1975-77 HEI systems, the tachometer connects to the TACH terminal on the distributor for V8s, or on the coil for 6-cylinder, and to a ground. For 1978 cars, 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.


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

  1. 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.
  2.  

TDC or Top Dead Center corresponds to 0°; 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 special wrenches which make 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 information) to retard, and against the direction to advance.
  2.  
  3. Tighten the locknut. Check the timing, in case the distributor moved as you tightened it.
  4.  
  5. Replace the distributor vacuum hose, if removed. Correct the idle speed.
  6.  
  7. Shut off the engine, disconnect the timing light and tachometer.
  8.  

 
label.common.footer.alt.autozoneLogo