Idle switch is a common problem- and well documented!
The other issue with the 51mm bodies is the butterfly chamfer on the edges sometimes doesn't provide an adequate seal with the throttle bore, or the shaft binds slightly with the machined housing, or the return springs are full of crap, or the idle switch doesn't engage.
Either way, digifant will try to control idle speed by retarding/advancing the ignition timing, and can get stuck in a loop (hence the hunting), usually solved with a blip of the pedal.
Vacuum leaks post throttle butterfly also have similar symptoms, injector o-rings are a favourite here.