See Figures 1, 2, 3 and 4
Ignition timing is the point at which each spark plug fires in relation to its respective piston, during the compression stroke of the engine.
As far as ignition timing is concerned, the position of the piston can be related (in degrees) to the following reference terms: Top Dead Center (TDC), After Top Dead Center (ATDC), and Before Top Dead Center (BTDC). The movement of the piston is expressed in degrees due to the rotation of the crankshaft. Even though the crankshaft turns 720° to complete one entire 4-stroke cycle, all we're concerned about here is the compression stroke, since this is when the ignition of the air/fuel mixture takes place (or more accurately, should take place).
Because it takes a fraction of a second for the spark (at the spark plug) to ignite the air/fuel mixture and for the mixture to burn completely, the spark should ideally occur just before the piston reaches TDC. If the spark didn't occur until exactly TDC or ATDC, the piston would already be on its way down before the mixture explosion and would not exert as much downward force on the piston as it would if the ignition timing was properly set. The result of this would be reduced power and fuel economy.
Should ignition of the air/fuel mixture occur too far BTDC (advanced), the mixture explosion will try to force the piston downward before it can mechanically do so. This contest between the explosion forcing the piston downward and the crankshaft forcing the piston upward will result in a pinging sound if you're lucky; severe engine damage if you're not so lucky. If you experience pinging, check with a reputable
mechanic to determine if the pinging is mild or severe. Only a trained car mechanic can safely determine this.
Pinging can also be caused by inferior gasoline, since lower octane gas burns at a faster, more uncontrolled rate than a higher octane fuel.
In order to compensate for low quality gas, the ignition timing may be retarded a couple of degrees, though this is not recommended since performance and fuel economy will suffer.
On federal emissions engines, after the initial (base) timing is set, the computer and related components electronically determine and adjust the degree of spark advance under all conditions. On Canadian models, total ignition timing advance is determined by three things: initial timing setting, distributor vacuum control and distributor mechanical control.
See Figure 5
The timing meter picks up the engine speed and crankshaft position from the crankshaft balancer. It uses a luminosity signal through a glow plug probe to determine the combustion timing. Certain engine malfunctions may cause a incorrect timing reading. An engine malfunction should be corrected before a timing adjustment is made. The marks on the pump and adapter flange will normally be aligned within 1.27mm (.050 in.). A timing meter Tool J-33075 is recommended for this job.