If the gas is almost to the point of varnish it's time to shit can it, just looking for trouble even if you run it in a beater. But if it just smells bad and changed color you can use this stuff it will bring it back to life, won't change the color or take the smell away though but at least you can use it!..
PRI-G! (http://www.priproducts.com/index.htm)