1977 Models
The STC system will allow full spark timing only when carburetor throttle valve vacuum port is exposed to manifold vacuum, the thermal vacuum control valve is closed, vacuum switching valve is on, top gear switch is on, and when coolant temperature is below 53°F (12°C) or above 140°F (60°C).
When these conditions are met, atmospheric pressure to the distributor is cut off so that manifold vacuum can act to advance spark timing. When carburetor throttle plate is above vacuum port on carburetor body, manifold vacuum is applied to the distributor vacuum chamber. See Fig 1. This allows partial advance of spark timing whenever vehicle is accelerating or when cold starting.