A lot of sensitivity must be used, but the way you describe it, the car appears too rich. Since cars in control can not be anywhere near too rich I'm not sure.
The next step would be to grab the plate retaining nut with some long pliers and gently lift. if the engine smooths before getting too lean it would confirm the above.
To put a handle on this one should watch the duty-cycle control or EHA current to be sure there is an electronic reaction to the mixture corrections you are making.
Bosch Master, ASE Master, L1
33 years MB technician