Michael, MY 1970 280S only had one screw. A quick look at the old, ragged, 5% loose-leaf manual indicates that high idle speed is to be adjusted on the FRONT carburetor only, for thermostatic chokes. The spec was 2500-2700 rpm, warm engine, perfect idle (more about that in a sec), vacuum hose for ignitiion changeover removed from distributor (as of US model year 1970). You can only adjust the idle speed with the throttle wide open (to access the screw), so you must stop the engine for each turn of the screw. The book says 1/2 turn of the screw will get you 200-300 rpm (in=higher, out=lower), and says to repeat the check on the other (rear) carb only in case of carbs without thermostatically controlled choke valves. As far as a perfect idle, good luck!!! I found that my Unisyn carb flow meter was close enough, once I got the secondary throttle valves properly adjusted (probably closed), to balance the air intake for front and rear carbs. Irregardless, I found myself partially disassembling one or both carbs at least every couple of months, for a rough idle/idle miss, invariably caused by plugged idle jet (the long tube with an extremely small origice at its bottom). I'm not sure I ever conquered the problem, or identified its true cause. Dirt and an occasional spider would find their way into the float bowl, despite clean fuel and air filters, good gaskets, plastic plugs over the choke linkage,etc. Clearing the idle tube would always return the car to full, smooth power output at idle and off-idle. The only other early maintenance headaches I recall were the exhaust heat valves hanging up, but I believe new thermostatic springs and the right lubricant finally fixed that problem.
The factory service manual I have doesn't list 250S, but if you can't find anything else, make me an offer of less than $20, pay for shipping or pick it up in Herndon VA, and it's yours. Or try Rusty for the correct manual. I also have a chassis and or climate manual here as well for 114 and 115 chassis, of the same vintage, but in better shape. Neither of these two factory manuals has seen much use over the last 20 yrs. Mike Boerner 87 300sdl, 87 300d, 01 ml320
