you need to check that the shield by the rear of the engine that stops the water from the back tyre splashing on the ht lead is there,if its missing either make a new one out of a flap or buy a new sheld,it fits onto the engine shroud i think(black plastic).the reason will be water causing the ht lead and cap going to earth b4 its power gets to the spark plug.
adam bike doctor(of milton keynes motorcycle centre)
January 2011