Best I can tell from your pic, parts 4 and 7 combine to hold the throttle in place. 4 are fiber discs that work like brake pads or clutch discs. 7 applies pressure and is adjustable using the bolts that hold the stack together. I think if you tighten the bolts and pressurize the stack, the throttle will staywhere you leave it.
The spring might hold tension on the carb linkage assembly. But don't think it has anything to do with position control of the lever.