You got 50% this is what I understand. What is itr after hooking up the sensor? Does it go to zero percent (or close)?
If the value .876 is voltage at the O2 sensor lead, then you are rich. Hold the engine at 2000rpms and adjust the screw till the voltage drops. It will go from where its at to near zero in only a few degrees rotation of the adjustment from where it starts to change the voltage. You must remember that the O2 sensor is only good for telling actual mixture in the range 0-1.0% CO (tailpipe tested). You will run great all the way to about 7% CO so you could have to adjust considerable before you hit the appropriate range.
Bosch Master, ASE Master, L1
33 years MB technician