AN SMA INSTRUCTOR PLT RAN OUT OF FUEL ON A XCOUNTRY TRAINING FLT.

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DEPARTED SBP ON A VFR DUAL XCOUNTRY FLT WITH STUDENT IN L SEAT. COMPLETE PREFLT WAS CONDUCTED INCLUDING WX AND ACFT. FUEL TANKS WERE TOPPED OFF 1 HR PRIOR TO THE FLT AND VERIFIED FULL PRIOR TO DEP. PREFLT PLANNING SHOWED A 314 NM TRIP WITH WINDS AT CRUISE ALT FORECAST TO BE LIGHT AND VARIABLE. INCLUDING AN ADDITIONAL 5 MINS FOR CLB; THE ESTIMATED TIME ENRTE WAS CALCULATED TO BE 3 PLUS 10. THE ACFT FLT MANUAL CRUISE PERFORMANCE AT 75 PERCENT PWR IS FOR A TRUE AIRSPD OF 106 KTS; FUEL CONSUMPTION (LEAN) OF 5.4 GALLONS PER HR; ENDURANCE OF 4.1 HRS AND A RANGE OF 425 NM. A NORMAL DEP AND CLB WERE EXECUTED TO 6500 FT AND THE ENG WAS LEANED PASSING 5000 FT AGL AND UPON REACHING CRUISE ALT. AT LEAST TWICE DURING THE FLT; THE MIXTURE CTL KNOB VIBRATED TOWARD LEAN ENOUGH TO CAUSE ENG ROUGHNESS; AND WAS RESET. I REMEMBER CHKING IT TWICE AND RESETTING IT FOR CRUISE USING THE 'LEAN-TO-ROUGH AND ENRICHEN SLIGHTLY' METHOD; BUT PAID LITTLE ATTN TO HOW MUCH IT HAD MOVED FROM THE PREVIOUS SETTINGS. THE FUEL GAUGES WERE NOTED TO BE NEARING THE EMPTY MARKS AT APPROX 2 PLUS 0 HRS INTO THE FLT AND WERE CONSIDERED TO BE UNRELIABLE BECAUSE OF THE PREFLT CALCULATIONS. AN ENRTE DSCNT WAS ACCOMPLISHED APCHING RBL (DEST) ARPT WHEN THE ENG FAILED DUE TO STARVATION. AN UNEVENTFUL LNDG WAS ACCOMPLISHED ON A NARROW GRAVEL SVC ROAD ADJACENT TO AN IRRIGATION CANAL. POST-FLT INSPECTION SHOWED SLIGHT FUEL SEEPAGE EVIDENCE AT THE FUEL GASCOLATOR BUT NOT CONSIDERED SIGNIFICANT TO THE PROB. AFTER FURTHER INVESTIGATION; WE HAVE CONCLUDED THAT THE MIXTURE CTL CABLE DOES NOT HAVE A FRICTION DEVICE INCORPORATED IN IT; THE FRICTION THROUGHOUT ITS RANGE IS MINIMAL AND ALLOWED IT TO VIBRATE TOWARD THE RICH POS SUFFICIENT TO CAUSE A HIGHER FUEL CONSUMPTION AND ULTIMATE EXHAUSTION AFTER 3 HRS 20 MINS OF FLT. JUDGEMENT WAS BASED ON PRIOR EXPERIENCE AND FLT PLANNING DATA. I'VE SEEN VERY FEW FUEL GAUGES THAT ARE ACCURATE AND WAS LULLED INTO NOT BELIEVING THESE. MY PERSONAL STANDARDS ARE TO NEVER LAND WITH LESS THAN 1 HR OF FUEL AND I FELT QUITE COMFORTABLE WITH THIS LEG AND WAS QUITE SURPRISED AND INTENSELY IRRITATED WHEN THE ENG QUIT. BELIEVING THERE MUST HAVE BEEN TRAPPED FUEL IN THE TANKS AFTER THE ENG QUIT; I TRIED UNSUCCESSFULLY TO RESTART THE ENG AND EXTEND THE GLIDE THE 3 MI TO THE ARPT; AND NEARLY PASSED UP THE LAST GOOD EMER SITE. MY DECISION TO COMMIT TO AN OFF-FIELD LNDG WAS MADE AT APPROX 500 FT AGL.