Vacuum Advance Kit; Adjustable; 1975-81 GM HEI Distributors; Built In Stop; Incl. 0.09375 in. Allen Wrench/Instructions; • Allows you to change the amount of ignition vacuum advance for maximum part-throttle performance while preventing harmful effects of spark knock (detonation) • Dial in as much spark advance as conditions permit—add more advance under light load conditions to improve throttle response and fuel economy, or limit spark advance in high compression engines and high load conditions to eliminate knock • Built-in stop prevents the adjustment screw from "backing out" like other designs • Perfect companion to Moroso HEI Advance Curve Kit No. 72300, below