The enrichener knob is plastic and can sometimes break.
Several options can be had to replace the knob. It can be done without removing the carb from the bike.

This one was done with a “Mooneyes” Deco Dash knob p/n SPC8425 $8.00.
The ID of the knob was originally too small so it was drilled out to fit.
The tools below were used for the replacement.

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