I need help with my 2007 yz450f. The problem im having is after rebuilding the top end my bike doesnt want to start. It ran fine before i worked on it. The things i tried was removing the cams and turning the engine over by pushing the kickstarter with my hand, with all the valves closed it has good compression. So the piston and rings i installed are good... when i put the cams back in and time it correctly it turns over really easy and i cant seem to find tdc because all the air is escaping out of the cylinder head vent hose, and it doesnt start. I lined the punch marks to the edge of the head where the gasket goes and the mark on the magneto with the mark on the case. so i know its timed correctly. It seems like the valves are open during the compression stroke. i dont know what else would cause it to loose compression after i install the cams. I thought maybe i installed the cylinder head gasket or base gasket upsidedown but they're uni gaskets so couldnt be that and their new gaskets. It has spark and i went through the carb completely and its working fine. does anybody have any ideas on what i could do or have had this happen to them before, i really dont want to take it in to the shop because it ran fine before i tore in to it and because i only rode it once after i bought it. i didnt check the anything on the cylinder head(valve clearences, springs, ect), but it should be fine because i wasnt having problems like this before.