The black knob is an air bypass to adjust idle speed. The adjustment for mixture is within the fuel distributor/air-plate housing. Between the fuel distributor and the intake bellows there is a small hole in the housing. Originally there was a little rubber plug on a stalk closing that hole (with no plug unfiltered air can enter - but the plug is usually gone by now). The adjusting screw is a 3mm allen and the tool has to be long enough to reach down inside (Snap-on makes such a 1/4 inch drive socket). The allen receptacle is often filled/clogged with dirt - I give a good spritz of carb cleaner through a skinny tube to clear the hole for the allen hex -- and then a blast of compressed air. The allen needs to fit securely. Adjustments need to be made in tiny increments -- clockwise to richen -- anticlockwise to lean. Once the adjustment is correct it rarely needs fiddling. I have not touched my good running '80 245 in the 160k miles I've put on the car--now at a total of 285k. -- Dave