Rough idle culprit?

My car is idling somewhat jerkily, fluctuating maybe 100 RPM between 600-700. Jerkiness aside, that's low, so I set about adjusting the base idle speed to about 800.

I shut off the engine, then jumpered between the two pins on the ignition board socket as explained in the Haynes manual as well as in various archived posts. I restart and the car is purring right along at 800, steady as a rock!

Take away the jumper... back to the old bluster. Adjust the idle faster to compensate, no dice - still rough and the same speed.

This jumper short circuits the throttle positioner, yes? Does this simple evidence point to that part as either dirty or faulty?

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Roger
'85 Carrera 3.2 Coupe aka "Sputterbug"
http://www.rogerplacer.net/911/