Check the gasket, between the pulse port and the bore of the carb. The original gasket is very thin along that area, and can develop a tear, not easily seen. A new gasket can be made, with just a hole (instead of the slot), that lines up with the pulse port and the pulse port channel in the fiber block. This leak, into the bore, will make the higher RPM's very lean, if you can get above mid range, at all. It took me a while to figure this out, on my 40CC, which I believe is almost the same setup. The gasket on mine looked perfect, but when I applied a little pressure with my finger, I noticed the gasket give way, a good bit. Try it, and let us know. Good Luck!

Andy