Pressure test the cooling system. You can borrow a pressure tester from many auto parts stores. Advanced Auto Parts for one. But before that double ck that the radiator cap is fully locked on. Also double ck to be sure that a 7lb cap is correct for that application. Does the generator have an overflow bottle? If so what is the status of the level in the bottle? If it doesn't have an overflow bottle I would add one. Is the alternator belt tight? Not slipping on the coolant pump pully? Start with the basics.