I looked up some info from an old Chilton's snowmobile repair manual and for the Walbro WF it shows two screws on top. The one closest to the air intake side is the low speed needle screw and the one on the engine side is the idle air needle. Back both needles 1 1/4 turns out from lightly seated. Turning either or both in will increase idle rpm. Turn low speed needle in a quarter turn and check for acceleration from idle. Adjust needle screws as necessary for best acceleration and idle. Final setting should be no more than a half turn in from initial setting. There is also a throttle stop screw for adjusting idle speed. Apparently the WF carb has a fixed high speed jet just like Mikunis do. It is located under a brass hex cap located on the air intake side of the float bowl.
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