To read a timing mark diagram, first look for the mark located near the timing scale that indicates the current timing setting. The scale is marked with major marks at 0, 8 and 16 and minor marks at 4 and 12. Read all numbers as degrees in relation to the mark at the top center. For example, if a vehicle's preferred timing is 14, it would be read as 14 degrees before top dead center.
Check a vehicle's manufacturing specifications to determine its ideal timing. Correct the timing by turning the distributor to the correct setting. Do this slowly, as rotating the distributor quickly can take the vehicle well outside its timing range and do irreparable damage to the engine itself. After setting the timing correctly, tighten the bolt that holds the distributor in place without disrupting the distributor's new setting.
Timing on older engines is corrected with the use of a timing light. This light flashes with the rhythm of the ignition and shines lights on the timing marks in order to make them stationary as the engine rotates. The stationary appearance the light provides makes it possible for the person correcting the vehicle's timing to do so efficiently, as the light illuminates any adjustments and shows if the engine fires at the correct point.