its sounds like its running weak,see many answers below the main and/ or pilot jet may be blocked the fuel may be stale or contaminated with diesel or other fuel the carb and /or tank may have water in it the vaccum tap may be faulty the carb inlet manifold may have a big leak or the airbox may be falling off or leaking......start with cleaning the carb out and making sure the jets are clear then go from there.
adam bike doctor
February 2010