I recently took on the project of rebuilding a 2004 KX 125. I have completed everything except the motor. I am in need of some help or suggestions as to a problem and advice on another:

It runs great at low to mid rpm's. When you really get the rpm's up in the higher range there is what feels like a serious vibration coming from the motor. First thought was loose motor mounts. But nothing was loose. Any ideas what this might be?

I also pulled the cylinder and head to look at the crank. The crank looks brand new with no signs of getting to hot. It does how ever have quite a bit of side to side movement where the rod connects to the crank. I know there should be a little but this seems like a lot.

Any help would be appreciated.