What is this part?
- Thread starter Kansan
- Start date
Vaanth
Well-Known Member
The pictured item is the Orifice Spark Advance Control (OSAC) valve. The is an emission control device, introduced in 1973, that delays the distributor vacuum advance movement when transitioning from idle to part throttle during vehicle operation. The valve is placed in the vacuum line between the ported vacuum advance port on the carburetor and the vacuum advance on the distributor.
The hole covered with the round block off plate next to it is for the temperature control unit, that senses air temperature in the cowl, for the Exhaust Gas Recirculation (EGR) system, if the the car was equipped with it. If not equipped, the block off plate is in place.
The following images from the 1973 Service Highlights explain the OSAC system:
Thanks, Vaanth. Currently the vacuum advance is coming from the carb baseplate. Most of the emissions equipment has been removed from this car and I'm not really interested in putting it back on at the moment, if every. wonder if the make a block-off plate for this valve?
Kansan
Kansan
Vaanth
Well-Known Member
OSAC was required in all 50 states, but EGR was not in most states in 1973. So, for domestic applications, a block off plate was not used for OSAC. However, for export applications, a block off plate was available. Chrysler called it a cover, part number 3656646. Chances of finding one new are slim.
My 1973 Duster has the OSAC valve and the round cover. I removed the OSAC from the vacuum advance line when I got it in 1981 and thought about making a plate, but I happened to have an NOS OSAC valve on hand with nice markings, so I installed it and just ran dummy hoses with the end hidden. My round cover was tarnished, so I just painted it black.
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