The C-172 has not been getting enough exercise lately. The other night, Hubby took it up for its first launch since the annual in May. The fuel tests for both wings looked normal and blue, but the one under the belly returned what looked like yellow fuel. A second sample was the same. He eventually removed about a pint of the wrong colored stuff before getting blue fuel. Any idea what turns the fuel (100LL) yellow? Interesting side note: He was there with an old friend who is in the habit of simply pulling the knob and discharging some fuel from the gascolator without capturing it into a jar to test. If it had been his airplane, he would not have known that it was the wrong color.