My modified FX61 had needle valve problems for a long time. I tried replacing O rings, and even a new needle, I could not get ridof the leak. I could seebubbles in the hose from the needle to the carb. I took the stock needle valve assembly off of the backplate and replaced it with a remote needle valve attached to the plastic motor mount. Problem solved for good.
I agree with the previous poster, this is not a low rpm engine. I run either a 12x6 or 13x5 at about 14,200 on the ground slightly rich. Mine has a Performance Specialties piston and cylinder, ported crank, and larger carb. I run 15%, but I live at 5000 feet.