Fly wheel weight. Put the heaviest one you can find. It will help tremendously.
Make your 125 power smoother and easier to control with a Steahly flywheel weight. Soften the hit and make it more chuggable. Most woods riders will benefit as well as all beginner and timid riders. Easy bolt on installation, no drilling, grinding or welding. Consider getting the engine lock up tool (E-3) to make installation and removal even easier.
http://www.steahlyoffroad.com/yamaha...-weight-2.html