The only ones I've seen with no vent in the tank cap, have a separate air line entering the tank output fitting where the fuel line exits. Sometimes the air line meanders around the engine then has an open end - the meandering seems to be designed to prevent fuel from escaping from that air line. Others have the air line connect to the carburetor so that any fuel that escapes from it ends up going into the air intake.
If you have no air line, and no vent in the tank cap, you may need to consider the possibility that you have the wrong tank cap.
