I am redirecting my suspicion(original suspicion appears to have been verified) to the positioning of the idler shaft bearing etc.]
You hit the nail on the head!
I found the idler shaft key to be properly installed and locking the two sheaves. Still not clear why they were able to rotate seperatly before, but they are locked together now.
I found a 1/8" gap between the idler shaft mounting boss and the idler shaft that was causing the Speed control quadrant assembly to contact the idler sheave vane.
I atempted to adjust the idler shaft up towards the eccentric bushing by loosening the eccentric bushing boss bolt, however this only allowed the control sheave to move up and not the bottom sheave (idler sheave).
I checked for proper drive belt placement between the sheaves at max fast speed setting and it looked good as well as operated freely with no binding.
For now the fix will to be careful not to drive the speed control to max fast, thus eliminating the possiblity of jamming the idler sheave into the speed control quadrant.

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