well if the fuel is fresh and the carb jets are definately clean including the choke jet then it may be either a faulty auto choke or it may be low on compression,also if the tick over has been wound in too far it will not start as it gets too much air when it wants more fuel,so basically if you ahve turned the tickover up to get it to idle then the reason for having to turn it up may be a lack of compression and the open throttle will not let it start.
adam bike doctor(of milton keynes motorcycle centre), January 2011