Orifice Spark Advance Control (OSAC) System

Fig. 37 OSAC valve. 1982-83 RWD models:

The OSAC valve, Fig. 37, is incorporated to aid in the control of NOx (Oxides of Nitrogen). The valve controls the vacuum to the vacuum advance actuator of the distributor. A tiny orifice is incorporated in the OSAC valve which delays the change in ported vacuum to the distributor by about 17 seconds (10 or 27 seconds on some applications) when going from idle to part throttle. When going from part throttle to idle, the change in ported vacuum is instantaneous.
Vacuum is obtained by a vacuum tap just above the throttle plates of the carburetor. This type of tap provides no vacuum at idle, but provides manifold vacuum as soon as the throttle plates are opened slightly. Proper operation of this valve depends on air tight fittings and hoses and freedom from sticking or plugging due to deposits.