Carb needs dismantling and properly cleaning in carb cleaner/ultrasonic bath. Replace rubber 'O' rings on inlet pipe if they're loose.
That should cure the hunting. The speed is controlled by the tension on the govenor spring. If this is stretched or damaged, it will affect the engine speed (and therefore the self-drive speed).
phil_saunders@bigfoot.com
Phil Saunders
January 2009