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Vacuum Advance Mechanism Test

  1. Connect distributor advance hose to distributor. Increase engine speed to 2500 RPM and hold for 60 seconds. Check ignition timing with vacuum advance hose connected and then allow engine to return to idle. If ignition timing advanced, vacuum advance mechanism is okay. Go to VACUUM RETARD MECHANISM TEST below. If timing did not advance, go to next step.
  2. Using vacuum hose "T", connect vacuum gauge in vacuum advance hose. Increase engine speed to 2500 RPM and hold for 60 seconds. Check vacuum gauge for presence of vacuum and then allow engine to return to idle. If vacuum reading is at least 15 in. Hg, inspect vacuum diaphragm for leaks and/or pick-up assembly for binding. Repair or replace as necessary. After repairs, repeat step 1). If vacuum reading is low, go to next step.
  3. Check engine vacuum control valves and related vacuum hoses for proper operation, routing, or damage. Use vehicle's emission label as a guide. If valves and hose are okay, go to next step. If not, repair or replace as necessary.
  4. Disconnect and plug distributor vacuum advance hose. Attach a vacuum hose directly between distributor vacuum advance diaphragm and intake manifold (alternate vacuum source). Increase engine speed to 2500 RPM. Check ignition timing and then allow engine to return to idle. If timing advanced, check vacuum source. Remove vacuum gauge and repeat step 1).
  5. If timing did not advance, inspect vacuum diaphragm for leaks and/or pick-up assembly for binding. Repair or replace as necessary. Disconnect alternate vacuum source. Connect normal vacuum source and repeat step 1).
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