I picked up an '06 YZ250 today and it had a significant knock in the engine. I asked the guy if he ran premium because my '02 would knock really loud when running anything less than 95 octane. He said he did but I'm not sure if I believe him.. Just out of my own curiosity I torn down the bike and top end right when I got home.. It has a hod rod crank and the piston had significant wear and side to side slap/ play in the jug. Even though it still started first kick when I was looking it over before purchase and felt the compression with my hand which "felt good" I still think the top end needs to be done. How ever I am not sure if this is where the knocking is coming from. The bike would not stay idling even after fiddling with the adjustment screw on the carburetor. It also has a full FMF fatty system, stock reeds. I have not checked the jets yet... I can take pictures of the piston and jug if that would help. What is a good test/ indication to tell whether my crank rod bearing/ main bearings need to be replaced? Any help would be GREATLY appreciated so please give me any feedback, I open to all suggestions.