Sorry I got on the wrong bus there: when you said at the beginning that you'd replaced both the O rings, I took it literally. With the emulsifier O ring leaking, I would expect the mixture to have been rich, not lean, and the symptom would have been the engine speed "rolling", like a car with too much choke applied.
I think you should try to push the throttle linkage down to idle by pushing on it directly. Once you know for sure whether it is just a matter of the throttle being held open by the linkage, you'll be halfway there. The problem sounds as if it will probably be related to bigted's diagnosis - some part of the breather tube holding the throttle linkage - which you may be able to verify just by removing the tube completely and seeing whether the throttle butterfly will then return to the idle position.
