See Figure 1
Ignition timing is the measurement in degrees of crankshaft rotation of the instant the spark plug fires, in relation to the location of the piston (while the piston is on its compression stroke).
Although no periodic service is necessary, ignition timing can be adjusted by loosening the distributor locking device and turning the distributor in the engine.
Ideally, the air/fuel mixture in the cylinder will be ignited (by the spark plug) and just begin its rapid expansion as the piston passes top dead center (TDC) of the compression stroke. If this happens, the piston will be beginning the power stroke just as the compressed (by the movement of the piston) air/fuel mixture starts to expand. The expansion of the air/fuel mixture will then force the piston down on the power stroke and turn the crankshaft.
It takes a fraction of a second for the spark from the plug to completely ignite the mixture in the cylinder. Because of this, the spark plug must fire before the piston reaches TDC, if the mixture is to be completely ignited as the piston passes TDC. This measurement is given in degrees (of crankshaft rotation) Before the piston reaches Top Dead Center (BTDC). If the ignition timing setting for your engine is seven (7°deg;) BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 7°deg; before top dead center of its compression stroke. However, this only holds true while your engine is at idle speed.
As you accelerate from idle, the speed of your engine (rpm) increases. The increase in rpm means that the pistons are now traveling up and down much faster. Because of this, the spark plugs will have to fire even sooner if the mixture is to be completely ignited as the piston passes TDC. To accomplish this, the ECU unit incorporates means to advance the timing of the spark as engine speed increases.
If ignition timing is set too far advanced (too far BTDC), the ignition and expansion of the air/fuel mixture in the cylinder will try to force the piston down the cylinder while it is still traveling upward. This causes engine "ping'', a sound which resembles marbles being dropped into an empty tin can. If the ignition timing is too far retarded (after, or ATDC), the piston will have already started down on the power stroke when the air/fuel mixture ignites and expands. This will cause the piston to be forced down only a portion of its travel, resulting in poor engine performance and lack of power.
Ignition timing adjustment is checked with a timing light. This instrument is connected to the Number One (No. 1) spark plug of the engine. The timing light flashes every time an electrical current is sent from the distributor, through the No. 1 spark plug wire, to the spark plug. The crankshaft pulley and the front cover of the engine are marked with a timing pointer and a timing scale. When the timing pointer is aligned with the 0 mark on the timing scale, the piston for the No. 1 cylinder is at TDC of its compression stroke. With the engine running, and the timing light aimed at the timing pointer/scale, the flashes from the timing light will allow you to check the ignition timing. The timing light flashes every time the spark plug in the No. 1 cylinder of the engine fires. Since the flash from the timing light makes the crankshaft pulley seem stationary for a moment you will be able to read the exact position of the piston in the No. 1 cylinder on the timing scale.
There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is inexpensive. The second type of light operates from the battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.
For the vehicles covered by this manual, it is best to use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 plug wire, eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.