Sounds like you need to adjust your throttle cable tighter. There is a safety switch in the throttle block, well actually two. They are two spring loaded pins one or the other has to be pushed in for it to run. One pin is on the face where the throttle lever contacts the block. The other is in the pivot pin area. When you pull the throttle the pivot safety switch pin is pushed in and the face safety switch pin is released. When you let off the throttle the pivot safety switch pin is released and the face safety switch pin is pushed in. If your throttle cable is too loose when you let off the throttle it is not pushing the face safety switch pin in far enough.
Make sense?