1979 Models
When the throttle valve in the carburetor or throttle chamber is opened rapidly, the Vacuum Delay Valve (VDV) restricts the air flow in the line to reduce the rate of vacuum change. See Fig 1. The reduced rate of vacuum change provides the vacuum advance unit with some delay time. When vacuum from the vacuum source decreases, the vacuum advance unit responds normally because a one-way function is provided by the valve.