If there is no free play the throttle will stick. Its a double pull system, it Pulls to open and Pulls to close. If the two have no freeplay, it just stays where you put it.
Minor adjustments can be made at the throttle housing but this likely isn't what needs adjusted...
Down at the throttle shaft the return cable needs slack added. Determine which cable is which by just slightly rotating the grip. The first one to draw tight is the one doing the opening. Loosen the other one add free play and lock it down. Now adjust the primary so it has proper play during full turning L & R + fork compression. Getting this right now is important. Also confirm the adjuster at the grip is not out too far. If it is, thread it in some and make your adjustments below.
Now slowly adjust the return cable but NOT to a point of binding so you get the response desired and/or proper slack when OPENING the throttle. Think Trial and error.. .. ..