When I adjusted the metering screw in my SL, I would back it off, then start the engine and listen to it. Blip the throttle and see how the engine responded. If it sputters or hesitates, you've gone too lean. Make sure the engine is at operating temperature before adjusting anything. If the cold-start injector is still running it will throw off any adjustments you try to make.

I backed mine off to the point where it would sputter on throttle blip, then 1/8 turn CW. Go for a ride and romp on it. If you get hesitation, sputtering, or pinging, pull over and shut the engine off and go 1/8 turn CW again. Wash, rinse, repeat until you have the engine running properly as lean as it will go. Take it out and do some 50-60mph driving for a while and watch the temperature gauge. If it starts climbing, you may need to go slightly richer still.

Mine had misrouted vacuum lines. Mine is a Euro model and someone tried to plumb it like a US spec 380. There are enough differences that it didn't work out well. To compensate, there were several lines disconnected under the hood and the mixture was turned very rich. Once I corrected all the lines and installed new rubber boots on everything, it was so rich it was blowing black clouds. Once adjusted it runs like a top. No cat on mine, so it always has a smell of gas when cold.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)