I have an OS 46fx that doesn`t want to shut down, even when I`ve gone to idle and pulled the trim all the way down ( air intake appears fully closed ). It will rather slowly shut down and kill, but not like my other engines which shut down quickly. No problems in flight with it, however. Could this be an air leak ? Other possible causes? I suppose this is a better problem than having one that doesn`t want to run at all !!

A few suggestions for you. You can check if the carb Is seated all the way down against the engine. It has an O ring at the bottom and if its not seated snug agaist the case it could allow air in. On other engines the problem has come from a loose back plate, or one that won't seal without silicone, this usually isn't an OS problem however. I have a Tiger Shark 75 that won't shut down. I have learned a trick, if I hit the throttle open and then closed quickly it will kill the engine. The extra fuel combined with not enough air to burn it floods the engine and it dies. If you can't find the problem and you don't want to live with it you could call the store where you bought it as it may be a warranty issue and they may simply replace the engine for you.