Did you unplug the idle control valve to see what happens? The motor will slowly rev to around 1700 if its working. I agree with the tuneup strategy and make sure the timing is correct. How is the power otherwise? I know thats a dumb question for a 380, but can it at least outrun a Geo Metro? If its severely slow there may be other issues such as valves or rings.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma