I have a OS 46fx that I have ran for a while with no problems. The last time out I noticed it was slugish form idle to WOT. The low end was rich. I adjusted the mixture screw in to lean it out and restarted the engine, and the screw turned back to where it was.[X(] The screw is so loose that it is just turning be itself with the vibration.[:@] How do I fix this?