Every thing that ctaylor738 said plus my '76 280C would not idle if I tightened the air cleaner hold down too much as it would warp the carb and cause a vacuum leak.

If you get a chance to sniff the tailpipe as it's idling, or at least trying to and it smells sharp and makes your eyes burn, it's running way too lean.

If you get a chance to cover most of the air horn when it is warmed up and it begins to idle better, you are running lean.

My '76 450SEL tends to idle funny and run-on more if it's low on fuel, which has nothing to do with your problem but shows that low fuel level problems do exist.

Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120