well it could be a number of things,does it have good compression ?have you checked the crankseals for any leaks (you will usually see a oil leak as it looks black and sticky)also the woodruff key may have sheared and put the igntion timing out,have you checked the vaccum tap is working ok and have you cleaned the jets out properly in the carb?
adam bike doctor(of milton keynes motorcycle centre), January 2011