I just got done with replacing the top end and timing chain in my 2006 YZ450. It had about 60-70 hours on the engine and since I was the 2nd owner figured this would be cheap insurance. Everything went pretty well on the rebuild. There were a few lesson learned: 1. RTFM - More than once. When I put the cylinder and head on I over torqued the bolts. The method in the workshop manual calls for 22 ft lbs, then back the bolts all the way out, then torque to 14 ft lbs and then take each bolt another 180 degrees tighter. Seems simple, but I missed the 14ft lb part. Luckily I hadn't run the motor yet so I pulled the cams off and re-torqued properly. 2. If something doesn't Seem right / Feel right . . . It's probably not right. STOP AND FIGURE OUT WHAT IS WRONG. During one the multiple times I reinstalled the cams the exhaust cam holder seemed to go on a bit tough. The screws weren't as easy to thread in my hand as before and once torqued the valve clearance was off just a bit. I pulled the cam back off and discovered that the cam wasn't pushed all the way against the cam journal. (Away from the timing chain). Now typically the "C" clip would not let this happen but apparently the cam was just off by enough that the "C" clip was not pressing on it at all. 3. Once I got everything back together for the second or third time, the bike wouldn't start. I kicked and kicked. Of course this should never be the way it is so OBVIOUSLY somethig was wrong. So I once again pulled the cam cover off and checked timing. Guess what - It was off. So I re-aligned the cams and put it all back together. Still didn't want to start as easily as it did before the rebuild. Carb? Just tight after the piston? It would start if I gave it about 1/8 throttle and once started seemed to run pretty good. Would also start fairly easily when warm. But it still was bothering me so I ONCE AGAIN pulled the cam cover off to check the timing - which I was sure was fine. Well, guess what?? The Exhaust cam was still off one tooth. Even after I was sure it was right. So I pulled cams and re-aligned. BTW - I'm really good at pulling the cams now. Next valve check will take my like 10 mins After this final timing fix guess what? The Bike Started 1st KICK!!!!! So the bottom line is - If you work on your bike, especially around the timing, and the bike doesn't run, re-check what you did and pay extra special atention to the timing!