Thank you all guys
Ok I put a 13x8 prop as some of you have suggested me (and O.S. too) and he run much better , but still have problems after a couple of minutes of flight.
Like I said before, I already spend $200 on this high quality engine[>:], and I won't spend any more of my money on this engine, even on a simple needle valve at $19, it is supposed that it should runs well right out of the box. []

I will listen to the advice of AirGar, I will sell this motor and I will buy another of any other brand.

Again Thanks

BTW: before yesterday I was in my LHS and I saw the new 91FX, O.S HAD SOLVED THE PROBLEM!!!
The new 91FX has a NEW needle support, the same as that of the new 46AX