I noticed, that you mentioned, that it would run smoother with the EHA disconnected.

In that case, you should not attemt to adjust the mixture screw. With the EHA disconnected, the fuel distripbutor will operate in its "native" mode (as it did before the "E" version), where the fuel pressure increased proportional to the airflow by the mechanical control (air flow meter plate will move the plunger inside the fuel distributor). The mixture screw is adjusting this linkage, and since it appears to run OK with EHA disconnected, the adjustment must be close to the correct position.

The EHA is connected in parallel to the plunger to fine adjust the mixture according to the feedback from the oxygen sensor.

So, if it works well with EHA disconnected and ill when connected, the problem must be in the electronic loop. Either it could be the EHA itself (it does fail sometimes), or it is the control by the ECU, which basically gets input from oxygen sensor, air mass sensor, altitude sensor (if provided), coolant temperature, rpm, speed and throttle position switches.

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1991 300SEL (W140)
1987 190E 2.3 (W201)
1987 Honda Accord 2.0