carb need a clean maybe ? any other problems like flooding or carb overflowing ? all electrics working ?
most cases if she starts ok but then cuts out its a fuel problem ...wip of the carb and sit at a table with it on a old towel or sheet and strip and clean .
this relates to other carbs but same basic cleaning ideas http://knol.google.com/k/how-to-clean-a-motorcycle-carburetor-the-right-way#
only say this as my mates was doing the same other day started ok then splutterd a little and cut out after getting him to realy clean the carb it runs fine he also found reeds was cummed up with dirt or crap .
did you clean carb ? if so have you put everything back as it should be ? do the revs feel ok and not stuck open or jammed . did you adjust any screws on the carb