I have noticed the compression on my 2009 YZ450 is very weak. It doesnt take much force to kick it over. It has over 80hrs but it still starts easily. Does this mean piston needs to be replaced, but the valves may be good? Please excuse my ignorance as I am trying to learn proper 4 stroke engine maintenance . If my hypothesis is true, I was planing on doing a top end, which looks fairly straight forward per this tutorial but was wondering if I also need to replace valves and the timing chain as well. I will inspect the crank once I have it all apart to see if that needs replaced. Is there anything else I should consider replacing?