Well just remember that your LSN affects the high speed running as well. The HSN is designed to limit the ultimate fuel flow at top speed, provided your LSN isn't the actual limiting factor. I had my LSN set so that it idled and launched well, but at high speeds I could screw the HSN out all the way and it didn't do a thing to the smoke or temps. I cranked out the LSN so that it idles a bit splashy and is a bit sluggish on takeoff, but now it runs a whole lot cooler and I can actually see the HSN doing stuff now. I've got a header and a pipe, too.

I'm thinking that the VX18 LSN needle isn't tapered enough for really high RPM or the additional flow of some modifications.

I had a 2" long crack in my tank and I thought that it might be causing some of the leanness, but after I replaced the tank and checked for leaks, it didn't change until I screwed out the LSN some more.

In your case, it might be a chunk of dirt still stuck in the carb even after you tried cleaning it out. Or it could be the LSN - try it flush and see how that affects things.