new bike not starting help

hi.just aquired a 2003 fs650.the engine has been rebuilt and a fcr41 fitted.also dynoed to tune.when the bike arrived it started 1st kick hot or cold and ran sweet as a nut.the bike was to used as a track bike before i bought it so all of the wiring was taped up with lights removed.today i started to try and sort wirng out.because the electric start has been removed the engine was started to check what wires were doing what.the engine was running fine and started 1st kick and was idling great and running for only a bout 90secs.i switched it off while i was sorting the wiring .while i was doing this there was a smell of petrol and a pool appeared under bike .the bike will now not start.every time i turn fuel on petrol seems to be coming out of exhaust headers!!.the carb is a mk 1 i think with no overflow pipes and no hot start. i removed carb and set it up with petrol tank attached to try to replicate but nothing.it looks as if the the float valve has stuck but nothing came out of carb when on bench but when put back on bike it came out of headers when i start kicking.is it just a stuck float valve ?so fed up new bike and all.any help would be great thanks also there is a good spark