1975-76 Type 2; 1971 and Later Type 4 With Automatic Transaxle
This vacuum operated assembly slightly opens the throttle when the transmission is put in drive to prevent the engine from stalling under the load.
To test, adjust the regulator as described in Tune-up . When the plunger clearance is correct, pull the vacuum hose off the idle speed regulator: the plunger should extend and open the throttle slightly. When you reconnect the hose, the plunger should pull back. If not, check the hose for blockage or rips, or replace the regulator.