Here is a carburetor with 19mm intake for the MotoTec Mud Monster Go-Kart. This fine little replacement carburetor has a 3/4" (19 mm) diameter intake, a manual choke, and a fuel shut off valve.
This style and size carburetor is typically found on Honda clone GX100 style horizontal shaft engines used on a wide range of kid-size go-karts, mini bikes, and all sorts of custom projects.