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 1000 rpm compression spring is specifically for the Yamaha 125’s.
Fits the following:
Zuma 125 / BW’s 125