Diesel Bombers - W115 Mercedes 300D: Mystery Knob

Okay I'm becoming intimately familiar with the fuel system in my 300D. I've already upgraded to the W123 style fuel filter. But in my amateur bumbling, I accidentally turned a knob on my fuel injector pump. I have NO idea what it does, and i can't seem to find any reference to it any where. It is labeled in ft. 2000 ft, 4000 ft, 8000 ft, and 12000 ft. I'm not sure what it does or where it should be set. I do notice that the higher i set it, the harder the start. Hopefully some one here can shed some light on it.

here's a link to a picture (i cant' seem to get the img tags to work) http://picasaweb.google.com/CarpeMor...39006442705794

thanks for the help.