I was annoyed by the vibration of my KLX331 (was a 250), so while I had the motor apart i decided to improve the crank balance. I calculated what was required and added the extra weight by drilling the crank and balancer and pressing in tungsten carbide rods (they are twice the density of steel). I put some spot welds on the ends to make sure they stay there. here are photos of the crank and balancer. The bike is now as smooth as it was as a 250. Not a job for the beginner, but it can be done. Dave.