See Figures 1, 2, 3 and 4
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each cylinder. Timing is usually measured 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 begin 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 plug must fire slightly 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.
If the setting for the ignition timing is 5º BTDC, each spark plug must fire 5º before each piston reaches TDC. This only holds true, however, only when the engine is at idle speed.
As the engine speed increases, the pistons move faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.
With both the Point Type and Duraspark ignition systems, 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. As the engine speed increases, the vacuum level also increases and in turn advances the timing. It is necessary to disconnect the vacuum lines from the diaphragm when the ignition timing is being set. Otherwise the timing marks will advance as the engine idle speed increases.
With the TFI-IV system, ignition timing is calculated at all phases of vehicle operation by the TFI module.
If the ignition is advanced too far, 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, the piston will have already passed Top Dead Center (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 is best checked with a timing light. This device is connected in series with the No. 1 spark plug. As the current flows down the spark plug wire, the timing light to flash reacts to the current by flashing.
There is a series of notches on the crankshaft pulley of the engine which represents a scale of degrees of crankshaft rotation. These are called timing marks. A pointer on the engine block helps identify the marks on the pulley. When the timing light is pointed at these marks while the engine is running, the pulsing light will identify a given timing notch on the pulley. By adjusting the position of the distributor, the timing of the vehicle, and their corresponding notches on the crankshaft pulley will change accordingly.