The idle speed control may need to be adjusted and/or lubricated, and there is another trick: Hold the accelerator down halfway while turning the idle knob counterclockwise. It should move easier this way as it takes the tension off the knob itself. When turning it clockwise, simply turn the knob and it should move easily (you can actually watch the accelerator pedal release slightly).

I always wondered, too, why the turbo models don't have the knob...perhaps there is a cold start module/solenoid that automatically speeds up the idle when cold? The Peugeot diesel I used to drive had this...I remember when it stopped working

-AC
'76 240D