Idle speed is electronically controlled; or should be.
At the front of the aircleaner, underneath is the cylendrical idle valve. Disconnect the electrical and the engine should rev to 2000rpm or more. If it doesn't rev, then the valve is stuck or the passage orifaced. Common bypass to the electronic control problem is to put a piece of broom stick in the pipe with a 4mm hole bored through it. I have found three or four.
There is no adjustment. If the valve is actually being controlled to such low speeds its likely a control unit problem.
Bosch Master, ASE Master, L1
33 years MB technician