8KCAB PLT DIVERTS DUE TO LOW FUEL CAUSED BY MISSING FUEL CAP DURING AEROBATIC MANEUVERS.

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DURING FUELING PRIOR TO THE FLT; THE R TANK FUEL CAP WAS EITHER NOT TIGHTENED OR NOT REPLACED. THERE WAS APPROX 2 HRS OF FUEL EVENLY DISTRIBUTED BTWN THE 2 TANKS. DURING AEROBATIC MANEUVERS; INCLUDING INVERTED FLT; FUEL WAS VENTED OUT OF THE R FUEL TANK. WHEN THE ENG SPUTTERED DURING INVERTED FLT; THE AIRPLANE WAS ROLLED TO STRAIGHT AND LEVEL; AND THE FUEL GAUGES CHKED. BOTH TANKS READ EMPTY. SINCE THERE IS A XOVER LINE BTWN THE TANKS; WHICH CANNOT BE SHUT OFF; THE VACUUM CREATED BY THE MISSING FUEL CAP ON THE R TANK EMPTIED TO L TANK FIRST. I INITIALLY LOCATED THE NEAREST ARPT USING MY GPS AND VECTORED TOWARD IT. SEEING THAT IT WAS ONLY A DIRT STRIP WITH NO FACILITIES; I LOOKED FOR ADDITIONAL ARPTS THAT MIGHT BE WITHIN RANGE; BUT THERE WERE NONE. THE CLOSEST WAS 17 NM S; AND I DID NOT HAVE CONFIDENCE THAT THE ENG WOULD RUN LONG ENOUGH. AS I ASSESSED THE DIRT STRIP; I NOTICED 2 PICKUP TRUCKS PARKED ON 1 END; BUT COULD NOT MAKE OUT PEOPLE. IN ORDER TO ENSURE SOMEONE KNEW WHERE I WAS; I MADE A 'MAYDAY' CALL. I DID THIS BECAUSE THIS AIRSTRIP WAS IN A VERY DESOLATE AREA AND I WANTED SOMEONE TO KNOW WHERE I WAS IN CASE I WAS STRANDED WITH NO FUEL. A PLT ANSWERED AND I RELAYED MY FUEL SIT AND INTENT TO LAND. THE LNDG WAS UNEVENTFUL. I RELAYED THAT I WAS SAFELY ON THE GND AND THAT THERE WERE PEOPLE WHO SHOULD BE ABLE TO PROVIDE ASSISTANCE. UNBELIEVABLY; ONE OF THE GENTLEMEN OWNED A CITABRIA THAT HAS AN STC FOR AUTO FUEL; AND HE HAPPENED TO HAVE FUEL AND A PUMP IN HIS PICKUP TRUCK. HE ALSO HAD A ROLL OF SPD TAPE; WHICH WAS USED TO COVER THE OPEN TANK. WE PUT 10 GALLONS OF AUTO FUEL INTO THE L TANK AND I DEPARTED TO A NEARBY ARPT TO ADD 10 GALLONS OF 100LL TO THAT; AND RETURNED TO ZZZ WITHOUT INCIDENT WHERE I DISCOVERED THE MISSING FUEL CAP VERY CLOSE TO WHERE I STARTED THE AIRPLANE AFTER FUELING. CONTRIBUTING FACTORS: THIS SIT COULD HAVE EASILY BEEN AVOIDED BY VISUALLY DOUBLE CHKING THAT THE FUEL CAPS WERE BOTH REPLACED AFTER REFUELING. THIS IS A NEW AIRPLANE TO ME (OWNED FOR LESS THAN 1 MONTH) AND A NEW OPERATING ARPT WITH A SELF-SERVE FUEL ISLAND. IT WAS THE FIRST TIME I HAD USED THE FUEL ISLAND AND THERE WAS SOME FRUSTRATION INVOLVED WITH OPERATING THE SYS; WHICH PROBABLY DISTR MY ATTN FROM THE IMPORTANT THINGS AS WELL. MECHANICALLY; THE DESIGN OF THE FUEL XFLOW SYS MAKES THE ACFT VERY SUSCEPTIBLE TO UNEQUAL DISTRIBUTION OF PRESSURE; AND WILL FLOW FUEL TO THE LEAST PRESSURIZED TANK. BECAUSE THE PLT DOES NOT HAVE THE ABILITY TO SHUT OFF THE XFLOW; EVEN IN LEVEL FLT; WILL VENT OUT OF A TANK WITHOUT A FUEL CAP; EMPTYING THE TANK THAT IS CAPPED VERY QUICKLY. HAD I NOT EXACERBATED THE PROB BY PERFORMING AEROBATICS AND LITERALLY DUMPING FUEL OUT OF THE TANK; I MOST LIKELY WOULD HAVE NOTICED THE PROB IN TIME TO RETURN TO ARPT WITHOUT INCIDENT. I WOULD HAVE STILL HAD NO CTL OF THE XFLOW FROM THE CAPPED L TANK; OTHER THAN TO SLOW MY AIRSPD TO LIMIT THE FUEL VENTING. THE ROOT CAUSE OF THIS INCIDENT WAS MY FAILURE TO VISUALLY AND PHYSICALLY CONFIRM THAT THE FUEL CAPS WERE IN PLACE AND TIGHT. IN THIS ACFT; PHYSICALLY CHKING REQUIRES A STEP LADDER; WHICH IS STILL EASIER TO FIND THAN AN AIRSTRIP OR A FUEL TRUCK IN THE MIDDLE OF NOWHERE.