All 1973 models equipped with 5.8L, 6.6L, 7.0L or 7.5L engines manufactured before March 15, 1973 are equipped with the Delay Vacuum Bypass (DVB) spark control system. This system provides two paths by which carburetor vacuum can reach the distributor vacuum advance. The system consists of a spark delay valve, a check valve, a solenoid vacuum valve, and an ambient temperature switch. When the ambient temperature is below 49°F (9°C), the temperature switch contacts are open and the vacuum solenoid is open (de-energized). Under these conditions, vacuum will flow from the carburetor, through the open solenoid, and to the distributor. Since the spark delay valve resists the flow of carburetor vacuum, the vacuum will always flow through the solenoid when it is open (this is the path of least resistance).
When the ambient temperature rises above 60°F (16°C), the contacts in the temperature switch (which is located in the door post) close. This passes ignition switch current to the solenoid, energizing it. Once energized, the solenoid blocks one of the two vacuum paths. All distributor vacuum must now flow through the spark delay valve.
When carburetor vacuum rises above a certain level on acceleration, a rubber valve in the spark delay valve blocks vacuum from passing through the valve for from 5 to 30 seconds. After this time delay has elapsed, normal vacuum is supplied to the distributor. When the vacuum solenoid is closed, temperatures above 60°F (16°C) are vented to atmosphere. To prevent the vacuum that is passing through the spark delay valve from escaping through the solenoid into the atmosphere, a one-way check valve is installed in the vacuum line from the solenoid to the distributor.