About a month ago, I dropped an exhaust valve. I then bought a WR426 head, brand new Wiseco Piston, Intake and Exhaust HotCams, and a new spark plug. Just to clear it up, it is a YZ426 bottom end on a WR426 head and WR426 valves. Valves were professionally shimmed. The only thing from the WR426 is the head and valves, rest is either new or from the original YZ426. The engine is now built, torqued to spec, and kicks over easily. Problem is, it will not start. I have spark, fuel, and air. Carb was not touched, other than taken off and put back on. It will backfire every so often, and it is extremely loud, never heard my bike do this before, but it shows I have combustion. There's some theory's I have, such as the pilot jet in the carb, since it sat for a month it could of got clogged. If you have any suggestions, please let me know. Thank you guys, I'll be in touch!