the ldle speed control valve may be sticking or failed,common problem on cozzies,is sits in the throttle housing and is held in by 2 screws its about 2 inches long and about an inch in diameter and has a connector pluged in the top and has a stainless body.
remove the connector,take out the two screws and pull the valve out of the housing,it may be stuck in tight but it will come out
when removed examine the end part as it may be dirty.simply wash the end part in petrol until its clean also look down into the throttle housing and make sure theres no bits of subaru,s in there
before you reinstall the valve put a small bit of vasiline on the o-ring to ease instalation,then bolt and reconnect up and see what happins.
to check if the valve is working,before you install it plug the connector into it and turn on the ignition you should see the end part move