The timing light will illuminate the pointer over the vibration damper. The timing is checked by pointer the light at the timing marks. Every time the number one plug fires, the strobe lamp lights. Each time it fires with the damper in the same position in relation to the pointer, the damper timing mark looks through it were standing still. To adjust the timing, the distributor clamp is loosened and the distributor is turned by hand. As it is turned, the timing mark will move. When turned in the proper direction, the mark will line up with the pointer. When the two are aligned, the engine is properly timed and the distributor clamp can be tightened. Remember to reconnect any vacuum lines or electrical connectors as applicable.
Magnetic Timing Meter.
Many late model engines can be timed with a magnetic timing meter. This meter has a timing probe which is installed in a magnetic timing receptacle near the conventional timing marks. The timing meter also uses an inductive pickup which clamps over the number one spark plug. Once all connection are made, the engine is started, and timing can be read directly from the meter dial.
Timing Advance Mechanisms.
As engine speed increases, it is necessary to fire the mixture sooner. If this is not done, the piston would reach TDC and start down before the air-fuel mixture can be properly ignited. To properly fire the air-fuel mixture charge, a device is needed to advance the engine timing (firing more degrees before TDC) as the engine speed increases. It is also necessary to retard the timing to control exhaust emissions and prevent spark knock. When the engine is at idle, very little advance is necessary. At higher engine speeds, it is necessary to fire the mixture somewhat sooner. To see this concept, look at the next figure, the pressure of a burning fuel charge will end when the piston reaches 23 degree after TDC. Notice in figure A, that the combustion cycle must start at 18 degree before TDC in order to be complete by 23 degree after TDC. In figure B, engine speed has tripled. It is now necessary to ignite the charge at 40 degree before TDC in order to complete combustion by 23 degree after TDC. The three common methods of advancing the ignition timing are centrifugal advance, vacuum advance, and electronic advance.