I have a Coleman Maxa 5000 ER Plus Generator that I purchased in 1992. It still runs good but the gas cap breather hole squirts gas out whenever it runs. I purchased a new cap but the replaced cap does the same thing. If I loosen the cap, the gas spills out around the threads. This problem exists whether the fuel tank is completely full or nearly empty. Any guesses as to what I can do to solve this problem?