On that 46 fx if its new make sure you run about 4 tanks thru it before you fly. After you run 3 tanks you will most likly have to reset the low in needle, when you do only trun it about 1/16 in and give it a try, I have never had to go over 1/8 of a turn to put the carb in sinc. for that motor. that will stop that stumble a mid range. also I like to use Slimline mufflers when I can..
Pat in Las Vegas NV>