Festiva L4-81 1.3L SOHC EFI (1989)
Distributor Advance Unit: Description and Operation
Centrifugal Advance
Centrifugal advance of spark timing, with increasing engine speed, is achieved by rotation of the rotor and armature shaft in relation to the distributor
shaft. The rotor shaft is driven by the distributor shaft. The two are connected through centrifugal flyweights, pivoted on a plate which is an integral part
of the distributor shaft. Drive pins on the flyweights engage slots in a plate at the base of the rotor shaft. Springs pull the flyweights in toward shaft center
when at rest and at low speed, holding the rotor shaft in the base timing or full-retard position. As engine speed increases, centrifugal force causes the
weights to pull outward against the springs. At 1050 rpm they begin moving the rotor shaft in the advance direction.
The dual-diaphragm vacuum advance mechanism acts on the stator plate which is free to rotate in the distributor housing. The larger diaphragm chamber
is connected to the spark port on the carburetor, just above the throttle at the idle setting. The smaller diaphragm chamber is connected to the intake
manifold, below the throttle. The resultant advance response is based on throttle position, engine speed, and engine load.
High Altitude Spark Advance
The high-altitude spark advance correction is all-electronic, and does not affect the relative positions of armature or stator. The ignition module goes into
high-altitude mode when barometric pressure drops below 670 mm-Hg (26.4 in-Hg). On carbureted vehicles, barometric pressure is calculated using
input from a manifold absolute pressure sensor mounted on the bulkhead to the right of the steering column. EFI vehicles are equipped with a barometric
pressure sensor housed in the ECA. The normal delay circuitry of the ignition and engine control modules is then bypassed to advance the initial ignition
timing to approximately 6° BTDC.
