BUILDER; OWNER; OPERATOR OF AN APPARENTLY UNIQUE HOMEBUILT ACFT SUFFERS ENGINE FAILURE AND LANDS IN A FIELD.

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I DEPARTED IN MY AIRPLANE TO RETURN TO MY HOME BASE. DURING CRUISE AT APPROX 2500 FT MSL; THE ENG LOST PWR BUT DID CONTINUE TO RUN. THE ENG WAS MISFIRING SO BADLY THAT I COULD NOT MAINTAIN ALT. SUBSEQUENTLY; I COMPLETED A SUCCESSFUL OFF FIELD LNDG IN THE CTR OF A FIELD THAT IS USED TO GRAZE CATTLE. I WAS NOT INJURED IN ANY WAY; THE ACFT WAS NOT DAMAGED. I SECURED THE ACFT AND AFTER PHONE CALLS TO FRIENDS AND FAMILY I BEGAN TO WALK. IN A FEW MINS A LCL FARMER PICKED ME UP AND DROVE ME TO WHERE MY WIFE MET ME AND TOOK ME HOME. THE ACFT WAS MANUFACTURED BY ME AND ITS FIRST FLT WAS OCT/XA/99. SINCE THEN I HAVE FLOWN 785 HRS WITHOUT INCIDENT. THERE ARE 2 FUEL TANKS; AN 8 GAL TANK IN THE FUSELAGE AND A 40 GAL TANK IN THE WING. DURING THE HOT SUMMER MONTHS I INSTALL WINGTIP EXTENSIONS WHICH CAN CARRY 5.5 GALS EACH. DURING THIS FLT THE LONGER WINGTIPS WERE INSTALLED. THE INSTALLED ENG MONITORING SYS INCLUDES A FUEL TOTALIZER WHICH HAS PROVEN TO BE VERY ACCURATE. THERE IS ALSO A CAPACITANCE MEASURING PROBE IN EACH TANK. THE DATA FROM THESE PROBES HAS PROVEN TO BE REMARKABLY INACCURATE EXCEPT WHEN THE TANK IS NEAR FULL OR EMPTY AND THE ACFT IS STATIONARY AND LEVEL. 5 DAYS EARLIER; I KNEW THERE WERE 15 GALS OF FUEL IN THE WING TANK WHEN I MET MY INSTRUCTOR FOR A FLT REVIEW. THERE WAS NO FUEL IN THE FUSELAGE TANK AND I CANNOT RECALL THE LAST TIME I HAD PUT FUEL IN THAT TANK. WE FLEW TO ZZZ1 WHERE I ADDED 30 GALS OF FUEL INTO THE WING TANK. WE THEN COMPLETED A 75 MIN FLT THAT INCLUDED 3 TKOFS. ON TUE AND WED; MY WIFE AND I FLEW TO ZZZ2 AND BACK TO ZZZ. EACH FLT WAS 50 MINS. THU MORNING AT ZZZ; THE TOTALIZER INDICATED 13 GALS ON BOARD WHICH WAS CONSISTENT WITH WHAT I BELIEVED TO BE THE FUEL CONSUMED DURING THESE RECENT FLTS. BEFORE I LEFT ZZZ1 THE TOTALIZER INDICATED 10 GALS ON BOARD. THE GPS INDICATED THE DISTANCE TO ZZZ WAS 56 NM AND I BELIEVED THAT I COULD COMPLETE THE FLT ON 3 GALS OR LESS IF I DID NOT CLB BEYOND A MINIMUM CRUISE ALT. THEREFORE; I WOULD HAVE AT LEAST A 45 MIN RESERVE ON LNDG AT ZZZ. WHEN THE ENG BEGAN TO RUN POORLY; I TURNED ON THE BOOST PUMP WHICH SEEMED TO RESOLVE THE PROB. IN A FEW MOMENTS THE ROUGH RUNNING RETURNED. I DID NOT SWITCH TANKS BECAUSE I KNEW THE FUSELAGE TANK WAS EMPTY. I TURNED IGNITIONS ON AND OFF. I CHANGED MIXTURE AND THROTTLE AND PROP BUT THE ENG CONTINUED TO POP AND BANG. I DO NOT RECALL ANY ROUGHNESS OR SHAKING BUT THE ENG DID NOT STOP OR GET QUIET. I RESET THE GPS TO NEAREST ARPTS. THE 3 NEAREST WERE ALL OVER 10 MI AWAY SO I FOCUSED ON A LCL LNDG SITE. AFTER I WAS ON THE GND; I CONFIRMED THAT THE ELT HAD NOT BEEN ACTIVATED BUT IT DID SOUND ON MY RADIO WHEN I TURNED BOTH ON. I DID A CURSORY INSPECTION OF THE ACFT AND IT APPEARED TO BE UNDAMAGED. THE NEXT DAY I RETURNED TO THE SITE WITH AN A&P MECH TO TRY TO DETERMINE WHY THE ENG FAILED. I SUMPED THE WING TANK AND CLEAN FUEL CAME OUT. WE TURNED ON THE BOOST PUMP BUT IT DID NOT PRODUCE FUEL PRESSURE. AMAZINGLY TO ME; WE FOUND THE FUSELAGE TANK WAS ALMOST FULL OF FUEL. AT THE TIME THIS SEEMED TO BE AN IMPOSSIBLE FINDING. NEVERTHELESS; WE STARTED THE ENG AND IT RAN FINE ALTHOUGH I WAS RELUCTANT TO PRODUCE REAL PWR IN THE LOOSE DIRT. THE NEXT DAY; SATURDAY; WE RETURNED WITH A TILT-BED TRAILER; A BIG TOW TRUCK AND 10 STRONG AVIATORS. WE LOADED THE ACFT ONTO THE TRAILER AND MOVED IT TO A SMALL PVT ARPT ABOUT 5 MI AWAY. ON SUNDAY; I RETURNED TO THE ARPT WITH 2 A&P MECHS AND SEVERAL OTHER KNOWLEDGEABLE AVIATORS. TOGETHER; WE DID A COMPLETE CHKOUT OF THE ACFT. EVENTUALLY; WE DID SEVERAL RUN-UPS INCLUDING A FULL PWR RUN OF 2 MINS. AFTER THAT; I HAD AN UNEVENTFUL FLT BACK TO ZZZ. BACK IN THE SHADE OF MY HANGAR; WE HAVE STILL BEEN UNABLE TO FIND ANY DAMAGE TO THE ACFT. THE 'BRAIN TRUST' (MY EAA AND A&P FRIENDS) HAS FOUND THAT UNDER CERTAIN CIRCUMSTANCES OF HIGH ALT OR HIGH TEMP WITH MORE THAN 16 GALS OF FUEL IN THE WING BUT LESS THAN FULL; IT IS POSSIBLE FOR FUEL TO BE FORCED BY AIR PRESSURE INTO THE FUSELAGE TANK. I NOW BELIEVE THIS ANALYSIS IS CORRECT AND ITIS LIKELY THAT DURING THE 100 DEG DAY ON THE GND IN ZZZ2 WITH ABOUT 23 GALS ON BOARD THAT FUEL DID MIGRATE INTO THE FUSELAGE TANK. HAD I KNOWN THAT IT WAS POSSIBLE FOR FUEL TO BE IN THE FUSELAGE TANK WITHOUT MY HAVING PUT IT THERE; I WOULD HAVE SWITCHED TANKS AND IT IS LIKELY THAT THE FLT WOULD HAVE BEEN COMPLETED SUCCESSFULLY. IT IS UNLIKELY THAT THIS PROB WILL RECUR SINCE I NOW KNOW THAT FUEL CAN MOVE INTO THE FUSELAGE TANK. NEVERTHELESS; THE POSSIBILITY IS UNACCEPTABLE TO ME AND I WILL CHANGE THE VENT SYS TO ELIMINATE THIS CONDITION.