I just finished a 2014 YZ250F that had all new oem cylinder head , valves , springs , cylinder , mains , I rebuilt the crank ! It did not start cold after just a month and half, their problem was dirt getting past the filter ! Ruined the new valves fast !!!!! I would take a close look at the intake boot after the filter, You should be able to see if has been eating dirt ! IMO there is nothing wrong with YAMAHA oem valves, never switched one to stainless valves !Yes for some reson the guy over look the valves. The intake valves were very tight. I'm going to order some staneless intake valves