Since I did the rebuild on my W126 M103 the idle has been at around 550 to 600 RPM, it's not as smooth as I like it to be. When I push it to 700/750 RPM it smoothens out and the oil pressure gauge goes up to 1/4. How do I set the idle speed to 700/750RPM, where I believe it should be on these engines?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

idle is controlled by the idle control valve, no adjusting. from the manual-

http://www.w124performance.com/service/w124CD1/Program/Engine/103/07.3-2006.pdf

There is indeed a neat way to increase the idle on the 103 engine.
It's sometimes referred to as the "purple wire" trick. I did mine, temporarily, about 10 years ago, so I can't remember the details, but a search using that term should turn it up, or look under some of my old posts. Sorry, Old-age syndrome.
By the way, it turned out the rough idle at 600-700 was the infamous right motor mount failing for the tenth time. I didn't suspect that because the mount at the time was only a few months old - but the aftermarket parts on the right side under the exhaust can collapse in only a couple weeks. iI'd check that first.

DG

Will check, thanks. The motor mounts for the 300SE M103 are expensive. The set I bought from Mercedes was $300.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).