Date: 202101
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Takeoff / Launch
Airspace: Class G Y
Aircraft component: Indicating and Warning - Fuel System
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Flight Engineer / Second Officer; Instructor
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Training / Qualification
Second reporter function: Trainee; Student
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Inflight Shutdown
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: The subject aircraft was a newly rebuilt/restored EXPERIMENTAL Super Cub clone with a zero time engine and 35' tundra tires. The aircraft has less than 10 hours since the rebuild and had a couple of maintenance squawks; one of which was inoperable/unreliable fuel gauges.Our previous six flights were with a known fuel quantity and subsequent visual inspection which is a little problematic with the pitch attitude of the aircraft on the ground with the large tires. Fuel is observed in the tank only when the tank is mostly full.One of the fuel gauges appeared to indicate fuel during our first couple of flights which led us to believe that; at least through part of its indication range; would give a valid fuel indication. That was a false assumption.During pre-flight inspection on the previous day; the right fuel tank fuel drain broke and needed replacement. To empty the right tank of fuel; the fuel selector valve was placed in the 'OFF' position and fuel drained from the tank and collected in a six gallon container. The first several gallons removed from the right tank were used to completely fill the left tank. As fuel was drained it became apparent that the left tank was also draining because we learned that placing the fuel selector valve in the 'OFF' position does not prevent fuel transfer from one tank to the other. The fuel valve was then placed in the 'LEFT' position which does prevent fuel transfer and the right tank completely emptied. It was estimated that there was at least half a tank of fuel remaining in the left tank. Also; the left fuel gauge indicated what we expected to see. We were wrong.The aircraft was pre-flighted for our flight on the day of the incident and on the fourth landing and subsequent go-around; the engine lost power and was landed in an adjacent field off airport without incident. No injury to the two persons on board; no damage to the aircraft or property. The aircraft was towed back to the airport to investigate the cause. The gascolator screen was inspected and cleaned and flow rates judged to be normal. Further inspection revealed less fuel in the tanks than we originally thought.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Flight Instructor flying experimental aircraft reported fuel exhaustion inflight.