See Figure 1
The octane selector is used as a fine adjustment to match the ignition timing to the grade of gasoline being used. It is located near the distributor vacuum advance unit, under a plastic 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 truck at 16-22 mph 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 vehicle accelerates, the sound should go away.
- If the pinging sound is loud, or if it fails to disappear as the vehicle accelerates, 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).
On 1973-79 models, do not turn the octane selector more than1/2turn toward R. Do not turn it toward A at all.
- When the adjustment is completed, replace the dust cover.
One graduation of the octane selector is equal to about 10 degrees of crankshaft angle.