See Figure 1
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. Ignition timing is controlled by turning the distributor in the engine.
Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, this piston will be beginning the power stroke just as the compressed and ignited air/fuel mixture starts to expand. The expansion of the air/fuel mixture then forces the piston down on the power stroke and turns the crankshaft.
Because it takes a fraction of a second for the spark plug to ignite the gases 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 TDC and the full benefit 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 that 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 a means to advance the timing of the spark as the engine speed increases. In some vehicles that were made before 1972, the advancing of the spark in the distributor was accomplished by weights alone. Others have a vacuum diaphragm to assist the weights. It is necessary to disconnect the vacuum line to the distributor when the engine is being timed.
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 engine is 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.
The timing is best checked with a timing light. This device is connected in series with the no. 1 spark plug. The current that fires the spark plug also causes the light to flash.
When the engine is running, the timing light is aimed at the marks on the engine and crankshaft pulley.
There are three basic types of timing lights 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 close 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 car batterytwo 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.
Connect a tachometer to the BID or SSI ignition system in the conventional way; to the negative (distributor) side of the coil and to a ground. HEI distributor caps have a Tach terminal. Some tachometers may not work with a BID, SSI, or HEI ignition system and there is a possibility that some could be damaged. Check with the manufacturer of the tachometer to make sure it can be used.
Timing should be checked at each tune-up and any time the points are adjusted or replaced. The timing marks consist of a notch on the rim of the crankshaft pulley 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.
INSPECTION & ADJUSTMENT
Point Type Ignition
See Figure 2
Some early engines have 6v ignition systems. Make sure your tach/dwell and timing light have 6v capability.
- Locate the timing marks.
- Clean off the timing marks so you can see them.
- Mark the timing marks with a piece of chalk or white paint. Mark the one on the engine that will indicate correct timing when it is aligned with the mark on the pulley or flywheel.
- Attach a tachometer to the engine.
- Attach a timing light according to the manufacturer's instructions. If the timing light has three wires, one is attached to the no. 1 spark plug lead with an adapter. The other two are connected to the battery. The red one goes to the positive side of the battery and the black one to the negative terminal.
- Disconnect the vacuum line to the distributor at the distributor. Plug the end of the hose.
- Check to make sure that all of the wires clear the fan and then start the engine.
- If there is an idle speed solenoid, disconnect it.
- Aim the timing light at the timing marks. If the marks that you put on the pulley and the engine are aligned, the timing is correct. Turn off the engine and remove the tachometer and the timing light. If the marks are not in alignment, proceed to the following steps.
- Turn off the engine.
- Loosen the distributor lockbolt just enough so that the distributor can be turned with a little effort.
- Start the engine. Keep the cords of the timing light clear of the fan.
- With the timing light aimed at the pulley and the marks on the engine, turn the distributor in the direction of rotor rotation to retard the spark, and in the opposite direction of rotor rotation to advance the spark. Line up the marks on the pulley and the engine.
- When the marks are aligned, tighten the distributor lockbolt and recheck the timing with the timing light to make sure that the distributor did not move when you tightened the distributor lockbolt.
- Turn off the engine and remove the timing light.
See Figures 3 through 7
- 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 four or six) spark plug wire. Clean off the timing marks and mark the pulley notch and timing scale with white chalk.
- Disconnect and plug the vacuum line at the distributor. This is done to prevent any distributor vacuum advance.
- Start the engine and adjust the idle to 500 rpm with the carburetor idle speed screw on 1975-77 vehicles. On 1978 and later models, set the idle speed to the figure shown on the underhood sticker. This is done to prevent any distributor centrifugal advance. If there is a throttle stop solenoid, disconnect it electrically.
- Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley notch isn't aligned with the proper timing mark (refer to the Tune-Up Specifications chart), the timing will have to be adjusted.
TDC or Top Dead Center corresponds to 0°. B, or BTDC, or Before Top Dead Center, may be shown as A for Advanced on a V8 timing scale. R on a V8 timing scale means Retarded, corresponding to ATDC, or After Top Dead Center.
- Loosen the distributor clamp locknut. You can buy trick wrenches that make this task a lot easier. Turn the distributor slowly to adjust the timing, holding it by the base and not the cap. Turn counterclockwise to advance timing (toward BTDC), and clockwise to retard (toward TDC or ATDC).
- Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
- Replace the distributor vacuum line and correct the idle speed to that specified in the Tune-Up Specifications chart.
- Stop the engine and disconnect the timing light.