A C170 engine stopped from fuel starvation because the outboard tank in the right wing did not vent properly so when the tanks were filled only 16.2 gallons could be loaded instead of 25.

Narrative

I was flying a C-170 for a friend; who is only a student pilot; to show it to a potential buyer. I have flown this aircraft before and documented its fuel burn at 2500' on a 2.00 hr flight to be 6.8 gph. It has a fuel capacity of 37.5 gallons total; 12.5 on the left and 25.0 on the right. I use FLTPLAN.COM to plan my trips and it shows about 7.5 gph average fuel burn; which gives an endurance fuel of 5.0 hrs. I started this trip with a standard preflight including sumping the tanks and checking the oil prior to moving the aircraft out of the hanger. I topped off the fuel tanks myself and took off from a private field. I got flight following through Center and about half way through; Center advised me they lost my transponder. I turned it off and back on and stopped getting a reply light. I continued to my destination. There I pulled and reset the transponder CB; visited with the owner and had the aircraft topped off with fuel. It took 22.21 gallons so my 3.0 hour flight showed a burn of 7.4 gph. I flew to my next destination in just under an hour. On contact with Tower there I was getting a reply light but they were not receiving my transponder. I showed the aircraft to a man (he did not fly it) then departed for home. I was planning to continue through Class B Airspace before dark. I double checked my flight time and fuel burn and it looked fine but I thought since I was going to be landing after dark I might as well get some extra fuel. I had chosen an Executive airport because it was clear of Class B airspace and I would not have to bother ATC with my transponder problem again. Center cleared me direct to destination. As soon as I replied; my engine lost power. I turned on the auxiliary fuel pump and called 'Center I've power loss. What do you show as the nearest airport?' While making this call I pressed the NRST button on my 496. It showed a major airport 11.4 NM. Then Center called and said 120 degrees and about 11 miles. I replied 'that won't work--the engine has failed. I see what looks like a football field on my left.' As I bank to look; I pump the throttle and the engine regains power. I notice the auxiliary fuel pump pressure is 2.5 to 3--normal is 4.0. I realize I'm out of fuel and advise Center. While holding a left bank to keep the fuel flowing I started a climb and headed for nearest airport. The engine cut in and out as I tried to get all of the altitude that I could. I was advising Center all the way. They cleared me to land on any runway. I could see the runway and lights and had climbed to about 2700' when the engine quit running. I advised Center that the engine had failed and I was not going to make the airport. I saw two options; a bayou and a golf course. The bayou's banks looked steep; so it was my second choice. I started surveying the golf course to see if it was clear of people and to judge the distance of a clear way; keeping the bayou as an option and watching my altitude. Then; I saw a single person at the approach-end of a golf course. I committed to land and adjusted speed with slip and flaps. I had a normal touchdown; medium breaks; ending in a safe; normal landing. The propeller had stopped at about 500' AGL. My cell phone indicated the time of my phone call to Center to advise that I was safe and alive. The aircraft Hobbs meter showed only 3.4 hours since departing. About 3 hours of this was actual flight time. This would have been 11.02 gph burn for the full 3.4 hrs that the Hobbs indicated and 12.5gph for the 3hrs of flight. This led me to believe that the aircraft did not hold the full 37.5 gal that it was supposed to. After investigation; I decided to remove the wings of the aircraft and trailer it from the golf course to my airport. Two FAA investigators oversaw the metering of the fuel into the right wing and it turns out that the outboard tank did not vent properly. There are two 12.5 gallon tanks in the right wing for a total of 25 gal with a single fill port on the inboard tank. Itlooked topped off full at only 16.2 gallons. It turns out that the faster you fill the tank; the less fuel it takes to seem full; so when I showed full out I was about 12 gal short. Note the fuel gauges are on the inboard tank and showed full.