here is a forum for those... maybe you can find the answer there... sorry I cant help morehttp://www.allthingsmoto.com/forums/f-33/rm125-front-forks-24289/
a 125 is made for a person around a 130 pounds. You can adjust the preload by turning the great big nut on the bottom of the shock. You should have a bunch of thread left. Also if that doesn't work you can cut a link out of the spring and gring the end smooth so it fits in the holder. You can also adjust the rebound. That adjust the speed at which the shock works. Preload is a springs weight capacity and rebound is how fast the suspension works. If you slow down the speed at which it compresses that will help. When going through the whoops you want a slow rebound so you don't bounce.