Just for fun, you might want to put a vacuum gauge on it.

See,
http://autospeed.com/cms/A_2393/article.html
and
http://www.gregsengine.com/vacuum.htm

Or search for "diagnose engine vacuum gauge" if the links get blocked.

Also, be sure your ignition is up to snuff. A weak spark, incorrect dwell, a bad condensor and wrong polarity can cause problems at higher RPMs.

Michael

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