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.