1976 Models

When manifold vacuum is high, vacuum is supplied to retard side of distributor diaphragm to retard ignition timing. As engine speed and/or load increases, manifold vacuum increases and ported vacuum increases, this allows advance side of distributor to advance ignition timing.

When coolant temperature exceeds 203°F (95°C), thermo valve will open and vent intake manifold vacuum, stopping ignition retard and allowing engine to cool by advancing ignition timing.