Engine will not start?

The machine is a Husqvarna R53W, two years old. I have never had problems starting it & last used it two months ago. Now when I try to start it, it only fires briefly perhaps one in ten pulls and often kicks back in mid pull, yanking the pull cord back to the engine. I have a reasonable working knowledge of engines, have checked the spark plug and it sparks OK and is clean, and changed the fuel to ensure no contamination. I have noticed that when the lever is put to the 'Start' position and then moved to the 'run' position that the carb butterfly does not move but perhaps airflow operates this as I see it has a balancing spring? Have you any ideas what may be the problem? Many thanks