The flames could be from a lean pilot mixture. Flames in the exhaust are from unburnt fuel spontaneously combusting when touching hot metal in your pipe. I know it seems wrong but hear me out...Lean mixtures burn slow and incomplete so when you shut off the throttle the unburned fuel goes out the exhaust valve where it goes into a hot exhaust pipe that ignites it. You can normally fix it by fattening up the pilot screw a bit, maybe a quarter or half turn. That's just one option that it could be. Some other possible causes of after fire like you describe is an exhaust leak, or a vacuum leak or a leak somewhere that lets in air to the exhaust system. Good luck