No brian farting. The system has to much coolent in it. The cap holds 16lbs. of pressure and the coolant finds a way out. Through the cap, down the tube, into the resivor that is designed to catch the overflow. Once the engine and coolant cool down, it condenses creating a vacuum inside the engine which pulls the extra coolent in the tank back in.
To me, it sounds like it is working as intended.
FWIW, to much timing and/or a lean running carb will introduce more heat than normal and cause an issue. So... how is your timing? Good cap, rotor, coil, wires and plugs? Carb running to lean/rich? Old fuel?
Expensive but correctly mixed with heavy water, a detail missed or not cared about.
