ORIGINAL: blw
How old is the .46NT? Was this the original plug that came with the engine?
Mine always runs great. Peak your highspeed needle valve and then back off at least 300 RPM. Also, I would suggest trying an OS A3 plug in this particular engine. Some people will tell you that the OS 8 is the perfect plug, but it isn't for most Evolution engines. They like hotter plugs. You may find it solves your issues and gains quite a bit more power. I haven't tried the A3 yet because a friend gave me a few of the older type of Tower's plugs. I'll go to the A3 when those are gone. I ran the Evo .455 with a Saito SS plug, but I didn't see as good of an improvement as with those older plugs. The newer Tower's plugs seem to be colder and not much of an improvement, and is the same as the Saito plugs.
If you rebuild, check the tank and lines like Biged suggested.
It's very new. I have another one that runs great. It came in a Hanger 9 P-51 PTS. I run a Hanger 9 plug in it. I will put that engine on it before I open up the fuel system to see how it runs on the plane. I didn't have a hotter plug with me i'm going to the hobby shop now then to the field to see if it helps.