Idle speed on a 300E is controlled by the ignition module and is not adjustable. If it was up around 2000rpm I can only think of a kinked or out of whack bowden cable causing it, or a massive vacuum leak.
How did you get it down? If you adjusted the duty cycle to do it then it must be sitting on a massively rich or lean stall speed. Either that or it was so far out in the first place. Not good
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I would be looking for cracked or broken vacuum lines, especially the ones that run under the plenum chamber.