I'm assuming that you mean the throttle tube won't return to its stop after you twist it ... and not something at the engine. And if you twist it back, it will close. If so, then here are a few things to look at.
Is the rubber flange on the throttle grip rubbing against the switch gear? Mine sometimes creeps over and drags.
If that isn't it, then check to make sure the springs are attached at the throttle body and nothing is binding there.
If you have recently been into the switchgear, then maybe a cable is out of place. Or maybe there is some corrosion if it has sat in a humid area.
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