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.
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 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 TDC (BTDC, or Before Top Dead Center). If the setting for the ignition timing is 5°BTDC, each 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 piston go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.
With the Dura Spark II system, 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 lines from the diaphragm when the ignition timing is being set.
With the TFI-IV system, ignition timing is calculated at all phases of vehicle operation by the TFI module.
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.
The timing must be 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 crankshaft pulley on 6-cylinder engines. A scale of degrees of crankshaft rotation is attached to the engine block in such a position that the notch will pass close by the scale.
On V8 engines, the scale is located on the crankshaft pulley and a pointer is attached to the engine block so that the scale will pass close by. When the engine is running, the timing light is aimed at the mark on the crankshaft pulley and the scale.
INSPECTION AND ADJUSTMENT
Dura Spark II Systems
With the Dura Spark II system, only an initial timing adjustment is possible. Ignition timing is not considered to be a part of tune-up or routine maintenance.
To inspect the adjustment, perform the following:
- Locate the timing marks on the crankshaft pulley and the front of the engine.
- Clean the timing marks so that you can see them.
- Mark the timing marks with a piece of chalk or with paint. Color the mark on the scale that will indicate the correct timing when it is aligned with the mark on the pulley or the pointer. 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 according to the manufacturer's instructions. If the timing light has three wires, one is attached to the No. 1 spark plug with an adapter. The other wires are connected to the battery. The red wire goes to the positive side of the battery and the black wire is connected to the negative terminal of the battery.
- Disconnect the vacuum line to the distributor at the distributor and plug the vacuum line. A golf tee does a fine job.
- 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 flywheel or pulley and the engine are aligned with the light flashes, the timing is correct. Turn off the engine and remove the tachometer and the timing light. If the mark are not in alignment, replace the ignition module.
With the TFI-IV system no ignition timing adjustment is possible and none should be attempted.