Distributor advance - checking
1 A secondary use of a timing light is to check that the centrifugal and vacuum advance functions of the distributor are working.
2 The tests are not precise, as would be the case if sophisticated equipment were used, but will at least indicate the serviceability of the unit.
3 With the engine idling, timing light connected and vacuum pipe disconnected and plugged, increase the engine speed to 2000 rpm and note the approximate distance which the pulley mark moves out of alignment with the mark on the scale.
4 Reconnect the vacuum pipe to the distributor and repeat the test when for the same increase in engine speed, the alignment differential of the timing marks should be greater than previously observed. Refer to the Specifications for typical figures.
5 A further check of the vacuum advance can be made by removing the distributor cap after the engine has been switched off, disconnecting the distributor vacuum pipe at its suction end, and sucking the pipe. The suction should be sufficient to move the distributor baseplate slightly.
6 If these tests do not prove positive renew the vacuum unit.
7 Some models are equipped with a spark delay/sustain valve in the vacuum line from carburettor to distributor, the purpose of which is to delay vacuum advance under certain part throttle conditions. If such a valve is suspected of malfunctioning, it should be tested by substitution, or taken to a Ford dealer for specialised checking. The main effect of the valve is to reduce exhaust emission levels and it is unlikely that malfunction would have a noticeable effect on engine performance.
8 If a ported vacuum switch (PVS) is fitted in the vacuum line, its purpose is to bypass the spark sustain valve when normal engine operating temperature (as sensed by the temperature of the coolant flowing round the inlet manifold) has been reached.