I have a 1980 280E, idles smooth, compression below ideal at 140-145psi, starts instintly. I have the idle set to 950rpm, but will drop to ~500rpm when put in drive. I haven't been able to find a vacuum leak, but I assume this must be the problem.
Any other thoughts on the cause?

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd