Let us get some terminology straight here!
The 'floating' sheave is the outer one on the motor shaft. It floats to maintain tension as the speed control is varied.
The other half is referred to as the 'fan' sheave. It does not move along the motor shaft and has the vanes for air circulation for cooling the motor. Hence the 'fan' designation.
The sheave which is moved by the speed control is referred to as the 'control' sheave. It moves along the idler shaft(the one in the middle between the motor shaft and the output(quill) shaft.
The other half is referred to as the idler sheave. It does not move along the idler shaft.
The speed control pork chop(quadrant gear) pivots on a roll(tension) pin inserted through two 'arms' extending into the headstock from the speed control mount. They have a tendency to get bent causing the wormgear to not mesh properly with the quadrant gear teeth.(I believe this is caused to some degree by turning the speed control down(to lower speeds) too fast. As a result of this mis-alignment, the teeth on the quadrant gear get damaged(the quadrant gear teeth are aluminum, the worm gear is steel).
I thought I had posted a link to a post describing this problem and one person's fix, but apparently I failed(forgot to 'submit'?) I will attempt to relocate it and add it to this post shortly.
http://www.shopsmith.net/forums/showpos ... stcount=15
As Dusty has indicated the 'high speed adjustment' is necessary to ensure proper movement of various parts through their design intended range. For an initial setting, adjust the adjustment screw so the end is flush with the top of the jam nut. Do watch the sawdust session Dusty has provided the link for.