The octane selector is used as a fine adjustment to match the vehicle's ignition timing to the grade of gasoline being used. It is located near the distributor vacuum unit, beneath a plastic dust cover. Normally the octane selector should not require adjustment, however, adjustment is as follows:
- Align the setting line with the threaded end of the housing and then align the center line with the setting mark on the housing.
- Drive the car to the speed specified in the Octane Selector Test Speeds chart in High gear on a level road.
- Depress the accelerator pedal all the way to the floor. A slight pinging sound should be heard. As the car accelerates, the sound should gradually go away. If the pinging sound is loud or if it fails to disappear as the vehicle speed increases, retard the timing by turning the knob toward R (Retard).
- If there is no pinging sound at all, advance the timing by turning the knob toward A (Advance).
Do not turn the octane selector more than 1 / 2 turn toward R. Do not turn it toward A at all.
- When the adjustment is completed, replace the plastic dust cover.
One graduation of the octane selector is equal to about ten degrees of crankshaft angle.