The RT, which is a clone of the Dellorto SHA. Any carb that you use will need to be tweaked (jetting, needle height, etc.) That said, I run the NT "speed" carb on my mildly modified 66cc and it runs well with a #62 jet and the needle clip in the top groove. They can be had for less than $20 on ebay. Carbs are just part of better performance, it's the total system that needs to work together in harmony. Just a carb change will most likely only net a small increase in performance. Get the stock system running optimally before you start throwing money at it.