Several guys have reported solving the leaning out problem by using a detached needle valve assembly. The K&B needle is only about $15. The attached needle appears to get too hot, causing the fuel to vaporize. They did not need to go to larger fuel tubing.

I wonder about the fuel tubing size issue. The orifice sizes through the needle valve and then the spray bar assembly are much smaller than the fuel tubing. It would seem that most of the fuel restriction would be taking place through these orifices and that larger fuel tubing would have very little benefit. Also, it is curious why the engine can get enough fuel at full throttle, but not enough at mid throttle.

Someone should try a different carb. on this engine, one with a built in needle valve. Let us know what you find out.