Ignition timing is the measurement in degrees of crankshaft rotation of the instant the spark plugs in the cylinders fire, in relation to the location of the piston, while the piston is on its compression stroke.
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 beginning 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) and ignited (by the spark plug) 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 top dead center (BTDC). If the ignition timing setting for your engine is seven degrees (7°) BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 7° before top dead center of the 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 engine speed increases.
The distributor in your Toyota has 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 large circular housing on the side of the distributor.
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 lowers 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.
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 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 and will result 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 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 on the front of the engine.
INSPECTION & ADJUSTMENT
- Warm up the engine. Connect a tachometer and check the engine idle speed to be sure that it is within the specification given in the Tune-Up Specifications chart at the beginning of the information.
- If the timing marks are difficult to see, use a dab of paint or chalk to make them more visible.
- Connect a timing light according to the manufacturer's instructions. If the light has three wires, one (usually blue or green) must be installed with an adapter between the No. 1 spark plug lead and the spark plug. The other leads are connected to the positive (+) battery terminal (usually a red lead) and the other to the negative (-) battery terminal (usually a black lead).
- Disconnect the vacuum line(s) from the distributor vacuum unit. Plug it (them) with a pencil or golf tee(s).
- Be sure that the timing light wires are clear of the fan and start the engine.
- Allow the engine to run at the specified idle speed with the gearshift in Neutral with manual transmission and Drive (D) with automatic transmission.
- Point the timing marks at the marks indicated in the chart and illustrations. With the engine at idle, timing should be at the specification given on the Tune-Up Specifications chart at the beginning of the information.
- If the timing is not at the specification, loosen the pinch bolt at the base of the distributor just enough so that the distributor can be turned. Turn the distributor to advance or retard the timing as required. Once the proper marks are seen to align with the timing light, timing is correct.
- Stop the engine and tighten the pinch bolt. Start the engine and recheck timing. Stop the engine. Disconnect the tachometer and timing light. Connect the vacuum line(s) to the distributor vacuum unit.