I have read a few articles about bikes not starting after a top end rebuild, but I have had my bike checked and re-checked about 4 times by a mecanic and my 2006 YZ450F will not start. I have replaced the piston, rings, cylinder, clutch, clutch basket, oil pump, all top end gaskets , timing chain, and all 5 valves. The bike has great spark, leak down test showed compression at 100%, electrical system was checked, timing issues were checked(looked at the flywheel, placed a tool in the spark plug hole to make sure it was at TDC), cleaned the carb and fule was flowing just fine, the values were seated properly....you name it and I feel like me and my mecanic has gone through just about everything. When we kick it it just give a bit of poping but will not kick over. Any ideas, as my bike should be mecanically sound after the rebuild by a professional mecanic.