See Figures 1 and 2
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 body 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, the piston will be beginning its downward motion of 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 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 top dead center (BTDC). If the setting for the ignition timing is 5º BTDC, the spark plug must fire 5º 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 a 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. It is necessary to disconnect the vacuum line from the diaphragm when the ignition timing is being set.
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 Top Dead Center (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 timing light to flash.
There is a notch on the front of the crankshaft pulley on the 4-134 engine. There are also marks to indicate TDC and 5º BTDC on the timing gear cover that will assist you in setting ignition timing.
On the 6-232 and 6-258, there is a mark on the crankshaft pulley and a scale divided into degrees. The 8-304, 8-350, 8-360, and 8-401 have the same mark and scale arrangement.
The 6-225, 6-226 and the 8-327 have the scale on the crankshaft pulley and the pointer mark on the engine.
When the engine is running, the timing light is aimed at the marks on the engine and crankshaft pulley.
ADJUSTMENT Point Type Systems
See Figures 3 through 7
- Locate the timing marks on the crankshaft pulley and the front of the engine.
- Clean off the timing marks, so that you can see them.
- Use chalk or white paint to color the mark on the scale that will indicate the correct timing, when aligned with the mark on the pulley or the point. It is also helpful to mark the notch in the pulley or the tip of the pointer with a small dab of color.
- Attach a tachometer to the engine.
- Attach a timing light to the engine, according to the manufacturer's instructions.
- Disconnect the vacuum line to the distributor at the distributor and plug the vacuum line.
- Check to make sure that all of the wires clear the fan and then start the engine.
- Adjust the idle to the correct setting.
- Aim the timing light at the timing marks. If the marks that you put on the pulley and the engine are aligned when the light flashes, the timing is correct. Turn OFF the engine and remove the tachometer and the timing light. If the marks are not in alignment, proceed with 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 wires 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. Align the marks on the pulley and the engine with the flashes of the timing light.
- 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 lockbolt.
- Turn OFF the engine and remove the timing light.
See Figure 8
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.
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 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.
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 electronic 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.
To check and adjust the timing:
- 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 trucks. 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 degrees. 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.