Mixture hasn't been adjustable on a warm motor since the onset of closed loop lambda control in 1980. The only thing that was adjustable on the K or KE cars was the base mixture setting. Once set the idea was to have equal ability to correct electronically both lean and rich. If you adjusted that correct base mixture richer the cars system will correct back to lambda (the theoretically exactly proportional mixture of 14.7 to 1 air fuel ratio that equals the equation HC + O2 = CO2 + H2O with nothing left. If you continually correct richer you will eventually go beyond the cars ability to correct.
Any car running rich is NO LONGER functioning. All controls have been over-ridden. Well, as it happens, many people do that and the manufacturers were forced to build systems that automatically correct base positions. All of the LH, HFM and ME systems built since 1992 have been totally self-correcting to a point. That point causes many problems on the early cars as the factor of correction was about 15% both sides of the original fuel calculation. Later cars are allowed with software to correct as much as 32% each way.
If you are really running systematically rich then something has overpowered the system. With a scanner this correction factor can be read and reset, but the amount of correction isn't adjustable. It is also possible that the correction factor could be within limits and single cylinders are the problem. The correction factor gives the guidance here.
Bosch Master, ASE Master, L1
33 years MB technician