found the answer
That second adjuster is called a resistance trimming plug. It is used for adjusting the idle speed. Position 1 is for setting it to 600 rpm, position 7 is for 740 rpm. The rest of the position are in the following link.
How to adjust idle speed on OM603?
1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
and few more american cars.