Thanks for all the replys fellas......I should have mentioned I am into Grass Karts and not Bikes....I use the XR200 because its a simple wee motor ,easy to work on and in a kart the power is very usable ....its just as fast as the 250 4 strokes and depending on driver skills will out perform 200cc 2 strokes.I have used Wiseco hi comp pistons,changed jetting and played with exhausts.I am going to lighten the flywheel and try some porting....in the main I am seeking any knowledge that someone may like to share.....cheers Shane