Typically the shift fork gets galled and the thickness of the fork ends lessens, which causes excessive gap within the gear fork groove. This allows only a partial gear dog engagement and allows the gear to jump out under heavy load. Once the gear dogs get rounded it's a quick progression of failure. I restored a 1973 Kawi H1 street bike and they are NOTORIOUS for a very sloppy gearbox and trashed shift forks and gear grooves. Luckily my trans was "just" starting to show signs of burnished shift forks. The best cure is to have the gears undercut. I had Paul Gast of "Fast By Gast" do my H1 trans. Outstanding job, and trans now shifts lightning quick ! Not sure if FBG does KX trans gears but might be worth a call, he does outstanding work for reasonable money. Good luck
http://www.fastbygast.com/
