See Figures 1, 2, 3 and 4
Ignition timing is an important part of the tune-up. It is always adjusted after the points are gapped (dwell angle changed), since altering the dwell affects the timing. It should also be checked to compensate for timing belt or gear wear on engines with electronic ignition at the interval specified in the Maintenance Chart. Three basic types of timing lights are available, the neon, the DC, and the AC powered. Of the three, the inductive DC light is the most frequently used by professional tuners. The bright flash put out by the DC light makes the timing marks stand out on even the brightest of days. Another advantage of the DC light is that you don't need to be near an electrical outlet. Neon lights are available for a few dollars, but their weak flash makes it necessary to use them in a fairly dark work area. One neon light lead is attached to the spark plug and the other to the plug wire. The DC light attaches to the spark plug and the wire with an adapter and two clips attached to the battery posts for power. The AC unit is similar, except that the power cable is plugged into a house outlet.
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. Ignition timing is adjusted by turning the distributor body in the engine.
Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just before the piston passes TDC of the compression stroke. If this happens, the piston will be beginning its downward motion of the power stroke just as the compressed and ignited air/fuel mixture begins to develop a considerable amount of pressure. The expansion of the air/fuel mixture then forces the piston down on the power stroke and turns the crankshaft.
Because it takes time for the mixture to burn, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be burned completely early enough in the downstroke 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 5º BTDC (5B), the spark plug must fire 5º 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, the distributor has a means to advance the timing of the spark as the engine speed increases. This is accomplished by centrifugal weights within the distributor and a vacuum diaphragm, mounted on the side of the distributor. It is necessary to disconnect the vacuum line from the diaphragm 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 there will be excessive temperature and pressure. 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 and creates less pressure in the cylinder, resulting in poor engine performance and lack of power.
The timing is best checked with a timing light. This device is connected in series (or through induction) with the No. 1 spark plug. The current which fires the spark plug also causes the timing light to flash.
The timing marks are located at the front crankshaft pulley and consist of a notch on the crankshaft pulley and a scale of degrees of crankshaft rotation attached to the front cover.
When the engine is running, the timing light is aimed at the marks on the flywheel pulley and the pointer.