how can i fix speedfight 50 2006 ac?

hi i have a speedfight 2 ac 2006 modle to start it i have to insert the auto chocke and then electric start it and hold the revs up and then remove the auto choke az well az keeping the revs up and then it will go but when i come to stoping it will cut out if i dont keep the revs up that is my problem it haz been to a garge and they have not got a clue can eney one help plz