Orifice Spark Advance Control: Operation

The OSAC valve delays vacuum spark advance during vehicle acceleration by restricting the flow of air from distributor vacuum advance chamber to carburetor vacuum port. During closed throttle deceleration or wide open throttle acceleration, the carburetor port vacuum drops below the level of vacuum in distributor vacuum advance chamber, and there is no delay in retarding the spark ignition timing. A one-way valve permits the vacuum advance chamber to respond to a lower vacuum instantly.

On California and high altitude vehicles, the OSAC valve is deactivated by a thermo valve when coolant temperature is 104°F (40°C) or below, in order to reduce HC emissions and to retain vehicle driveability after cold engine starting.