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 and is usually set according to TDC of the No. 1 piston. Ignition timing is 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 start the power stroke just as the compressed (by the movement of the piston) and ignited 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.
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). If the setting for the ignition timing is 5 degrees BTDC, the spark plug must fire 5 degrees before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, distributors have various means of advancing the spark timing as the engine speed increases.
The distributor in your Toyota probably has one or two means of advancing the ignition timing. One is called centrifugal advance and is actuated by weights in the distributor. The other is called vacuum advance and is controlled by that larger circular housing on the side of the distributor (models so equipped).
In addition, some distributors have a vacuum/retard mechanism which is contained in the same housing on the side of the distributor as the vacuum advance. The function of this mechanism is to retard the timing of the ignition spark under certain engine conditions. This causes more complete burning of the air/fuel mixture in the cylinder and consequently lower exhaust emissions.
Because these mechanisms change ignition timing, it is necessary to disconnect and plug the one or two vacuum lines from the distributor when setting the basic ignition timing. (Consult the emissions sticker under the hood of your vehicle. Follow those instructions if they differ from the ones listed here).
If 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 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 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 trave, resulting in poor engine performance and lack of power.
Ignition timing adjustment is checked with a timing light. This instrument is usually connected to the number one (No. 1) spark plug of the engine (see the equipment manufacturer's instructions). 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 its compression stroke. With the engine running, and the timing light aimed at the timing pointer and timing scale, the stroboscopic 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 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 light 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 car battery. Two alligator clips connect to the batter terminals, while a third wire connects to the spark plug with an adaptor. 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.
If your Toyota has electronic ignition, you should use a timing light with an inductive pickup. This pickup simple clamps onto the No. 1 plug wire, eliminating the adaptor. 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.