Deceleration System: Operation

During normal engine operation, intake manifold vacuum is applied at port 2 of the deceleration valve. Vacuum passes to the deceleration vacuum unit which pulls a link, opening the throttle flap to the accelerated idle position. If the throttle flap is already open past the accelerated idle position, the slotted link has no effect on engine operation.

When the throttle suddenly closes, maximum intake manifold vacuum is present at port 1 of the deceleration valve. The vacuum diaphragm contracts causing the barometric compensating diaphragm to contract which opens the two atmospheric ports. As the vacuum diaphragm contracts, it also moves the pushrod which unseats the check ball and allows manifold vacuum from port 1 to pass through port 2 and port 3. This holds open the throttle flap and advances the spark timing. See Fig. 2. 

As manifold vacuum decreases, the diaphragm expands, allowing the check ball to seat and close off vacuum to port 2 and port 3. At the same time, the barometric compensating diaphragm expands and closes the 2 atmospheric ports. The vacuum holding the throttle flap and advanced timing is bled off through the bleed-off port above the throttle flap. The throttle flap closes gradually due to the restricted opening of port 3. Timing returns to normal.