I just put a fresh piston in my 450 3 weeks ago. Since the new piston I races one motocross race the following weekend and it ran flawlessly. Yesterday I took it out to ride and when I unloaded it and kicked it there was no compression. I could "kick" it through with my hand easily. I put it in gear and rolled it forward with little resistance. I rolled it back and forward again and the lobe must have caught because all of a sudden the rear locked. I kicked it first kick and it fired. Never did it again the remainder of the day and I put 2.6 hours on it. Anyone ever experienced this? Could I have done something wrong in the rebuild? Should I look into getting a new cam?