Simple answer: reduce the air intake.
If the air screw is bottomed then the air is coming from somewhere else. Give a D-jet air and fuel will follow. The common place for additional air on that model would be a defective t-stat in the aux air valve (replaced as a unit - tall aluminum tower with two hoses at center front of engine).
After that I would be sure the throttle goes closed and lastly no vacuum leaks.
Bosch Master, ASE Master, L1
33 years MB technician