Ignition Timing

To adjust leading timing, loosen distributor lock nut, and rotate distributor housing until correct timing is obtained. See Fig 1.
To adjust trailing timing, loosen the screws securing the trailing vacuum unit. Move the vacuum unit outward (to advance) or inward (to retard). Retighten screws when correct timing is obtained.

Fig 1: Adjusting Ignition Timing. Distributor Position Determines Leading Timing, While Vacuum Unit Position Adjusts Trailing Timing.