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 (Before Top Dead Center or BTDC). 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 both the Point Type and Electronic Ignition systems, the distributor has a means to advance the timing of the spark as the engine speed increases. In some cases, this is accomplished by centrifugal weights within the distributor and a vacuum diaphragm mounted on the side of the distributor. On distributors equipped with a vacuum diaphragm, it is necessary to disconnect the vacuum line 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 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 timing light to flash.
Setting the ignition timing is basically the same for all engines. It is best set with a timing light. The simple 12v test light should not be considered a substitute for a timing light. Test lights are generally useful, however, for finding approximate settings after the distributor has been removed and ignition timing disturbed.
Before setting the ignition timing, be sure that the dwell is set to the proper specification, since this will influence the timing. This is not possible on electronic ignitions. If so equipped, the vacuum line at the distributor should be disconnected and plugged (a golf tee is handy for doing this) and the timing set while at idle speed.
It is a good idea to paint the timing mark with day-glow or white paint to make it quickly and easily visible. Be sure that all wires, hands and arms are out of the way of the fan. Do not wear any loose clothing such as ties, when reaching anywhere near the fan.
On engines with electronic ignition, your timing light may or may not work, depending on the construction of the light. Consult the manufacturer of the light if in doubt.