i love my 4-5 ys63's and had np with them. be sure your tank is set up rock solid with everything tight and no air leaks. set yur high speed needle to 1/8 turn richer then spec says. don't adjust anything else and by all means tighten your prop up...don't be afraid to tighten it. start the motor up and only adjust the high speed needle at high end....leave it rich a bit they love running rich. if thats a 12-6 APC prop it's fine. i use w---cat 20-20 in mine and they run great.[8D]