Ignition timing is the measurement (in degrees) of crankshaft position at the instant the spark plug fires. Ignition timing is adjusted by loosening the distributor locking device and turning the distributor in the engine.
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 (top dead center, the highest point in its travel), 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 10° BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 10° 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 distributor incorporates means to advance the timing of the spark as the engine speed increases.
On fuel injected vehicles there is no centrifugal advance or vacuum unit to advance the timing. All engine timing changes are controlled electronically by the ECU. This solid state "brain" ECU receives data from many sensors and commands changes in spark timing based on immediate driving conditions. This instant response allows the engine to be kept at peak performance and economy throughout the driving cycle. Basic timing and idle speed can still be checked and adjusted on these engines.
If the ignition timing is set too far advanced (BTDC), the ignition and expansion of the air/fuel mixture in the cylinder will try to force the piston down 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. This results 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 in the No. 1 cylinder is at TDC of it compression stroke. With the engine running, and the timing light aimed at the timing pointer and timing scale, the stroboscopic (periodic) flashes from the timing light will allow you to check the ignition timing setting of the engine. 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 to stand still for a moment, you will be able to read the exact position of the piston in the No. 1 cylinder on the timing scale on the front of the engine.
If you're buying a timing light, make sure the unit you select is rated for electronic or solid-state ignitions. Generally, these lights have two wires which connect to the battery with alligator clips and a third wire which connects to the No. 1 plug wire. The best lights have an inductive pick-up on the third wire; this allows you to simply clip the small box over the wire. Older lights may require the removal of the plug wire and the installation of an in-line adapter. Since the spark plugs in the twin-cam engines are in deep wells, rigging the adapter can be difficult. Buy quality the first time and the tool will give lasting results and ease of use.