I have written long descriptions of the procedures necessary to repair mixture problems on the inline mechanical fuel injection systems. Check for them and others in the archives.
I would say that this is definitely NOT a standard DIY affair but my decriptions give you every adjustment there is. Definitely enough info for an inexperience person without special tools to make a real mess of it.
If you really want to do this be sure that you understand the function and testing of the cold enrichment/additional air function. There is no way that internal adjustments should be made without an exhaust gas analyzer.
As to individual injectors, they can be bench tested by the equiptment designed for such. Most any Bosch Service Center should have such equiptment (and exhaust gas measuring tools).
Bosch Master, ASE Master, L1
33 years MB technician