Compression springs do a couple of things. First they supply the pressure to engage the belt. They also determine the rate at which the CVT transmission changes from low to high gear. A 1000rpm spring will go into a higher gear sooner than a 2000rpm spring.

This NCY 1500 rpm compression spring is specifically for the Yamaha Vino 125 & Zuma125 / BW's125.

Measurements:
Wire Diameter: 5.0mm
Inner DIameter:54.5mm
Length: 116.0mm