Remove the idle valve. It is at the front, top of motor, barely protruding from an installed aircleaner. Remove the aircleaner and the valve. Look in the hoses both directions and into both ends of the valve. You should be able to see the slide from one direction. The restriction will close off everything except a whole smaller than a quarter inch diameter (a quarter inch would probably run 2000rpm, an eigth inch hole will run too slow at about 500rpm.) The restriction will undoubtably be here as its so easy to get to.
Bosch Master, ASE Master, L1
33 years MB technician