The choke unit is controlled via waxstat you might want to remove it out and replace it the idea is a metal plunger moves in and out affecting the choke whne the engine is cold or hot if the waxstat is stuck in cold position the choke will not adjust itself and still thinks its cold hence over reving at idle.
I would look for Webber carb conversion then you can change over the whole set up for a more reliable manual choke system.
