When cold (oxygen sensor not yet warm, the engine control is operating
in open loop mode, i.e. it will regulate the mixture according to
various sensor inputs but without feedback from the oxygen sensor.
The basic mixture is adjusted by a sealed (plastic seal) adjustment
screw, The screw is located next to the throttle body and is accessible
through the long tube ("tower") from the top of the air filter
enclosure. The screw is spring loaded and must be pushed down to be
adjusted. It is factory preset to provide close to correct mixture
without ECU control, and should normally never be adjusted afterwards.
In fact this is how the non-electronic injection worked... the ECU with
EHA (ElectroHydraulic Actuator is a later add-on from K-Jetronic to KE
Jetronic (or CIS to CIS-E as Mercedes calls it). The mechanically
adjusted mixture will then be fine adjusted by the EHA according to
input from the ECU.
In your case, the mixture is obviously not correct in open loop mode,
and this adjustment could be the suspect (maybe someone adjusted it?).
Another cause could be a vacuum leak, but that would also be the case
in close loop mode when engine is warm, and it appears as if you
already checked this.
A fault in the EHA could create similar symptoms (if it is somehow
sticks near neutral position).
And another thing I can think of is the potentiometer, which gives the
Air Mass Sensor position input to the ECU. But that would probably also
give even worse problems when warm.