I had this exact problem with my TH .61. I tried loosening the carb and pushing down on the o-ring but still could not get it to shut down. Finally, I used a toothpick to spread a light coat of silicone on the carb barrell between the the o-ring and the engine case. I reseated the carb and tightened it while holding it down. I let the silicone cure overnight. That solved the problem. It runs great now, but as Minnflyer says, you will definitely need to adjust the mixture......high end anyway.