messing with the carbs shoudnt have had any effect, unless you kinked a line. i dont think theres all that much pressure in the cooling system. the radiator style cap is only rated for around 15 or 16 psi. if you can still read it whats it say?
havent a clue as to what would cause that valve to come apart, but it would create a loss of coolant or possibly an air pocket in the cooling system causing an overheat. and as mentioned, dont rule out the thermostat itself.