This is a friend of mine's bike (2007 Kawasaki KX250F) currently at my house for me to look at. He rebuilt the top end about a year ago and has never been able to get it to run right since. After putting it back together it was still making the same ticking noise it was before the rebuild, and then stopped running at all. He tried adjusting the valves multiple times but they will not stay in spec. What would cause the valves to not stay properly adjusted?