Hello I am new to four stroke single cylinders, i have a strange problem with a 1999 wr 400f. I bought the bike it had dropped a valve on to the piston so i have had new valves and seats put in the head and new piston and rings. I built the engine back up checked the valve clearances and the cam timing over and over but it still won't start every now and again it back fires through the carb but not all the time. I took the air box off and the exhaust tail pipe off when you kick it over both the intake and exhaust side suck and then blow every turn of the crank. I don't think this is right but I have never worked on a single cylinder only fours, any help or advice would be of great help, I am not letting an engine beat me.