On my Haynes manual, the task difficulty for any particular kind of service is rated on a scale of 1 to 5 (with 5 being the servicing best left to an extremely skilled technician).
Guess what, setting the fuel/air mixture shows up as a 5!
Not that the task is incredibly difficult, but that the diagnostic equpment required are probably only available to a reputable service shop, and it takes training to be able to use the equipment effectively.
This is not a trial-and-error shadetree mechanic kind of service...take it to someone who really knows what he's doing!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car