I've got a 20mm Keihin on my ped and I'm doing 50 Mph with only 49cc's. Before this carb I had a 16mm and before that a 12mm.
You need to precisely calibrate the carburettor by setting the airscrew to the right position, then checking if you need a larger or smaller main jet, then adjust the needle height and then resetting the airscrew to the correct position. If you do this correctly your bike will run great. Check the color of the sparkplug to see if the engine is running lean or rich.