Boost Controlled Deceleration Device
When manifold vacuum exceeds a pre-determined valve, (when throttle is suddenly closed) device provides an additional air/fuel mixture and delivers it below the carburetor throttle valve. This has the effect of delaying the drop of engine RPM. It should take 4 or 5 seconds for engine speed to decrease from 3000 to 1000 RPM. The amount of vacuum required to activate the device (and thus the time delay) can be varied by means of an adjusting screw in the end of the device.