Vacuum Retard

Adjust engine idle speed to 550-600 RPM. Using a timing light, observe ignition timing setting. Remove plug from manifold vacuum line and connect line to inner diaphragm. Timing should retard immediately. Replace dual diaphragm unit if either of the diaphragms are leaking or if the retard portion is out of calibration.

Fig 1: Dual Diaphragm Vacuum Advance & Retard Unit

Courtesy of FORD MOTOR CO.