Vacuum Retard Mechanism Test
- If vehicle is not equipped with dual (retard) diaphragm, ignition timing is okay. If dual (retard) diaphragm is used, go to next step.
- Disconnect and plug vacuum hose to vacuum advance diaphragm. Connect vacuum hose to distributor retard diaphragm. Check ignition timing at idle speed. If ignition timing is retarded below specification, ignition timing system is okay. If timing did NOT retard, go to next step.
- Using vacuum hose "T", connect vacuum gauge in vacuum retard hose going to diaphragm. Check for presence of vacuum at idle. If reading is at least 15 in. Hg, replace distributor diaphragm assembly and repeat step 2). If vacuum reading is low, check vacuum source and repeat step 2).
Courtesy of FORD MOTOR CO.