I hav an unusual problem with a O.S. LA40 when adjusting the High throttle. When closing the throttle the engine looses RPM's and keeps slowing down. When I go to close the needle valve the engine gains RPM's and keeps on gianing untill it gets too lean. I just stopped playing with it and flew the plane and it flew okey but there was a yellowish gas deposite all over the head. Is this engine ready to toss or is it okey.