Vacuum Spark Delay Valve: Description and Operation

Fig. 19 Spark delay valve--SDV:

This unit is used in conjunction with some of the other Ford systems. Its purpose is to further reduce emissions by delaying the spark advance during rapid acceleration and by cutting off advance immediately upon deceleration, Fig. 19.
This plastic disc-shaped valve is installed in the carburetor vacuum line at the distributor advance diaphragm. It is a one-way valve and will not operate if installed backwards. The black side of the valve must be toward the carburetor. This valve cannot be repaired. On all systems which employ the dual diaphragm distributor the line which has high vacuum at idle (normally operating temperature) is connected to the secondary (retard) side of the distributor vacuum advance unit. This is the connection closest to the distributor cap.