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It's called a Vernier Control. It has an internal screw that is part of the fixed housing, with threaded jaws that engage the threads around the outside and are part of the knob. When you turn the knob, you are turning the jaws with internal threads like a nut, and the whole thing moves in when you turn clockwise (richer) and out when counterclockwise (...


Probably because the actuation mechanism contains a screw drive which allows fine adjustment of the mixture- either that, or the screw drive locks the pull knob firmly in position so it will not vibrate out of the desired position.

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