Distributor Vacuum Control: Testing and Inspection

Fig. 20 Vacuum spark advance curve:

1. Warm up the engine fully.
2. Make sure that the idling speed and ignition timing are within specifications.
3. Gradually increase the vacuum on the vacuum advance diaphragm with a hand vacuum pump, note the amount of vacuum when advance starts.
4. When the vacuum is further increased, the relation between the vacuum and advance should be kept within range Fig. 20.