Before adjusting anything... I would make sure that the cable (both of them) and the linkage is set properly.
Down where the cable meets the engine... there is a cam, and a roller. At idle... there is a mark on the cam, and the roller should be lined up with it. Also... the roller should just be off the cam.
If they aren't lined up... then the throttle control may not be coming all the way back.
Also... there is a small cable that adds some RPM's to help in steering. That cable could have bound up, and is holding the throttle a little.