There are two little knobs that turn in increments (clicks) to reduce or increase the fuel flow to the carbs. One is attached to the long needle valve and the other to a small nylon valve in the top of the carby.
Having worked on Rotax engines for years and I never ever saw this. Please take a picture so I can have a look. There is the enriching circuit (choke) for cold starting only and sits to the side of the carb body and usually operated by a coupled cable..... unless he has this connected it to the knob you mention in order to richen the unit. Is there a primer circuit on the engine? If so then the chokes should remain permanently closed.