Okay so I have a 2009 yz450 as the title says and about 12 hours ago I put a piston and new timing chain in. I bought the bike used so I have no idea of actual hours on the engine, seller said he "thought" around 50 but wasn't sure. I've put 47 hours on it since June 2012. When I bought the bike and prior to tearing into the top end I checked the valve clearance and it was fine, just a little on the minimum side but not out of spec. So in the last couple riding hours it's gotten a little harder to start and so I thought I should check the valves. The exhaust side is fine, but the intake is very tight, way out of spec. I did clean a little carbon buildup off the valves while I had the head off, but even that wasn't bad. I did not disassemble the valves. So finally to my point, I'm gonna have to adjust some valve clearance and this whole shim and bucket is new to me. I understand the concept and I believe I need to order at least a shim kit, but A) any ideas why it tightened up so much in only 12 hours? And what type of shim kit should I look for and from whom? I'm assuming I just need to put a smaller shim in that will get me to the correct clearance? Any advice is appreciated, thanks.