Its the governor. The air vane moves the throttle. The tiny spring connects to the throttle linkage and the two screw bracket adjusts the speed. Since this type of saw might spend a fair amount of time running with no load, it needs the speed control to hold the max rpm down.
Its similar to the XLBC Brush Cutter but that uses a long throttle cable, not the trigger the circular saw uses. Here is the IPL for the XL-120, it has the governor parts shown. The XLBC throttle closes the throttle via a spring when it is released, the cable is not connected to the carburetor when the trigger is pulled. I don't know if the XL-120 works the same, allowing the governor full control of the engine speed when under load. I have never seen a Homelite circular saw in the flesh, either XL-100 or XL-120. Leon has the IPL for both but the governor parts are only listed on the XL-120.
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