when you say blocked with a screw, are they not recieveing any form of vacuum control atall now then?
as for idle, on the underside of the inlet, just below where the inlet boost pipe connects to the throttle, you will find a small adjustment screw recessed into the throttle, this controls tickover, will need a reasonable sized screwdriver to twiddle the screw.
cant remember which way raises or lowers the tickover, but atleast you know where to start now
and obviously set the tickover point once the engine is warm