It is there to adjust the idle a bit to smooth out the idle when you first start up in cold weather or if running the ac. You may never miss it. otoh you can surely buy the knob for not too much $. I let one go with the dash I sold a few months ago with the cable attached. They seem to be about 50 50 gone in the old 240s and euro 300s.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.