Hi, if the bike still has standard exhausts then it could be a weak mixture, find the air screw, turn it in (clockwise) slightly to richen the mixture.
Ted
July 2011
it sounds like its either got a leak in the exhaust system somewhere and or it may be running a bit weak.
adam bike doctor(of milton keynes motorcycle centre)
July 2011
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix motorcycles.