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5.3.
IGNITION SYSTEM VACUUM ADVANCE
5.3.1.
On standard points systems and some electronic ignition systems there are two types of advance methods used, both of which must
function correctly to obtain maximum performance and fuel economy.
5.3.2.
the first method is Mechanical or Centrifugal, which operates by the use of weights located in the base of the distributor. The weights
move outwards advancing ignition timing as engine RPM increases. This is tested by firstly removing the vacuum advance line to disable
the system, then with a timing light connected, run the engine rPm up checking that the timing advances in accordance with the
manufacturer's specification.
the second method is Vacuum Advance, which senses engine load via manifold vacuum. A vacuum diaphragm is mounted onto
5.3.3.
the distributor and connected to a rotating internal base plate which advances or retards timing as required to suit varying engine loads.
to test this system for correct operation, again with the timing light connected, raise the engine rPm and check timing advance against
manufacturer specifications. In the event that the vacuum advance is not operating, remove the vacuum line from the distributor advance
mechanism. Connect the vacuum tester (fig.2) and create a 5-10inHg vacuum, monitoring the timing at the same time. If a timing
advance is noted this confirms that the vacuum diaphragm and mechanical links are in order and that the fault is a vacuum supply.
To confirm this, connect the vacuum tester to the vacuum supply line and check the gauge reading. No vacuum should be noted at
idle but when the engine RPM is increased a vacuum increase should be observed. If this does not occur, trace the vacuum line back
checking for restrictions and breaks.
© Jack sealey limited
Original Language Version
VS4022.V2
Issue 1 17/01/18
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