Centrifugal and Vacuum Spark Advance

Centrifugal and Vacuum Spark Advance
(1985-1989 except Scirocco 16V)

Checking timing advance requires an adjustable timing light or an engine analyzer such as Volkswagen special tool VW 1367 to accurately read ignition timing throughout the range of timing advance. Follow the instrument manufacturer's operating instructions. See Specifications; Electrical; Ignition Timing, for centrifugal and vacuum advance specifications.
The centrifugal advance mechanism inside the distributor advances ignition timing with engine rpm. When checking centrifugal advance, first disable the vacuum advance. Disconnect the vacuum hose from the vacuum diaphragm on the distributor, and plug it.
Vacuum advance helps establish the basic ignition timing at low load when intake manifold vacuum is high. It gives less advance at higher load when intake manifold vacuum is low. To check vacuum advance, disconnect the vacuum hose from the vacuum diaphragm on the distributor and substitute a hand-held vacuum pump.
The vacuum advance diaphragm must not leak. It should hold about 500 mbar vacuum,losing no more than 50 mbar in one minute. Also make sure that the hose does not leak.
Also check the plate on which the Hall sender is mounted, It should move back and forth smoothly without sticking, and spring back to its original position when released. If not, then the plate is binding or the vacuum advance diaphragm is faulty.