Deceleration System: Operation
During normal operation, engine vacuum is applied at port 2 of the deceleration valve. It also passes to the vacuum unit, which pulls a link, opening the throttle plate. Since the throttle is already open, the slotted link has no effect on engine operation.
When the throttle suddenly closes, vacuum is present at port 1 of the deceleration valve. The bellows expands, unseating the check ball and allowing the vacuum also present at port 2 to pass through port 3, holding the throttle open and advancing spark timing. See Figure.
As manifold vacuum decreases, the bellows contracts, allowing the check ball to close off port 1 from 2 and 3. The vacuum holding the throttle plate and timing is bled off through the restrictor above the throttle plate. Timing returns to normal and the throttle plate closes.