All 1973 models equipped with 351C or W engines manufactured before 3/15/73 are supplied with this system. It provides two paths by which carburetor vacuum can reach the distributor vacuum advance. When the ambient temperature is below 49°F, the temperature switch contacts are open and the vacuum solenoid is deenergized. Under these conditions, the distributor receives full vacuum. When the temperature is above 60°F, the solenoid is energized, blocking the vacuum path. Distributor vacuum now must flow through the spark delay valve. When there is too much vacuum, the valve blocks distributor vacuum for 5-30 seconds by venting it to the atmosphere in a one-way valve. After this period, the distributor gets normal vacuum.