ORIGINAL: Pylonracr
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.
Bubbles from remote needle valves are not caused by o rings unless perhaps at idle. The fuel is under pressure, the O rings are there to prevent fuel from leaking out, not air in. The bubbles are caused by fuel vaporizing and are vapor bubbles not air bubbles. This is common to all remote needles and is not a probblem if plumbed so that bubbles do not collect and go through the engine at one time. Howver sometimes that is unavoidable and is why many manufactures are going back to needles mounted on the carb. Though the fuel will vaporize, it will vaporize where it needs to, in the carb.