First, replace the extension if there is any doubt about that. From there I'd check 100% that there is no binding in the linkage or that you are hitting a hard end point on the carb. I'm not familiar with your radio but is there a high speed mode? If so and if active, you may want to set the receiver to normal mode as the servo may not be compatible with HS. From there, though unlikely, you may just have a bad servo(s). I'd suspect binding or endpoints first.