check spark and see if it looks strong,check to see wat color the plug is.drain carburator with the drain bolt.Try LIGHTLY tapping the carb while its on the bike to make sure the floats not stuck(with a screwdrivers butt end).check the reeds because the symptons sound like what bad reeds would cause.The thing I think it will most likely be is the reeds,and the life of those will be dependent on alota stuff but if they havent been changed since you bought the bike thats probly the problem.And its gona sound stupid but make sure the gas is on.What oil ratio are you running?
Sorry for the long post but this is what you should check