Ok so I just bought this 09 yz450f. It's super clean and ok hours. Its had a recent top end including valves. It started 3rd kick when I was looking at it, and ran like a champ. I rode it also, plenty of power, and it didn't bog/want to die. There weren't any backfires/signs of valve clearance problems either. No abnormal sounds, ticks, ect. I drive the 2ish hours home, unload the bike, and now it won't start. It keeps flooding itself... so I take the tank, ect. off to get to the spark plug..and its wet as shit. I dry it off, wire brush it, and blow air into the combustion chamber to evaporate the fuel. Then I test the spark and its strong and blue. Then i put everything back together to find that within a few kicks the bike is backfiring and popping as I'm trying to start it. I am giving it zero throttle as I kick it also. It ran perfect before I hauled it home..and there is no way at all that it could've jumped timing. So I'm stumped. Probably going to tear it down after school tomorrow. Im thinking it could be a stuck float/needle valve..? There isn't any gas pissing out of the carb vent lines though. Thanks for everyone's help.