There's two carbs in question here I believe, and I think spookytooth has the best information on both, although they are both out of stock now.
the racing carbs beren around for awhile- at least I first saw it last summer- and it's the more expensive- I personally wish they'd leave off the choke cable and lever, and then they could also not ship a new twist grip, unless the cable is different or something-
if I could get the body by itself a little cheaper, I'd have probably tried to get one already- everyone has carb problems-
On the other hand, is the new NTS carb, and I think it'll eventually start shipping with newer kits as old stock diminishes- so caveat emptor- "let the buyer beware".
It claims ports 3mm larger, a larger slide barrel, and a larger main jet- the float bowl at the bottom appears about the same.
They are saying the older carb was designed for the 49cc motore and now these new ones are for the 66.