CH7A PLT CONDUCTED A FORCED LNDG WHEN ENG BEGAN TO SPUTTER FROM FUEL STARVATION.

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I DEPARTED AT XA09. MY SISTER WAS ABOARD AS PAX AND BTWN US HAD ABOUT 40 LBS OF BAGGAGE. I HAD TAKEN A FULL FUEL LOAD AT THE SELF SVC PUMP; JUST OVER 20 GALS. FUEL SEEPED OUT OF THE FUEL VENT UNDER THE L WING DURING TAXI TO RUN-UP; THIS IS NORMAL WITH FULL FUEL. 2 TANKS WITH 19.5 EACH 37 GALS USABLE. MY PLANE HAS A LYCOMING 0320-A2B. AT XD10 APPROX 6900 FT; AND 20+ MI OUT FROM ZZZ I NOTICED SOME SPUTTERING. I HAD THROTTLED BACKED THE ENG TO 2200 RPM AND NOW IT WAS WAVERING AROUND 1700 FT AND 1900 FT. I CHKED THE PRIMER; LOCKED. LOOKED AT TEMP AND PRESSURE BOTH FINE. I RAN THE THROTTLE BACK AND IT STABILIZED FOR AWHILE AT 1700 RPM. THEN MORE SPUTTERING; I HAD BEEN MONITORING ATC AND WITH MY SISTER ABOARD IT WAS A NO BRAINER; I CALLED A MAYDAY AND SQUAWKED 7700; RPTED MY POS AND QUICK GLANCED AT THE FUEL AND ESTIMATED 15 GALS (NORMAL FOR THIS LEG OF THE JOURNEY) WHEN ASKED. CYCLING THE THROTTLE GOT MORE RESPONSE BUT SPORADIC I PULLED BACK TO 20% AND SET FOR BEST GLIDE 60 IAS. MY ONLY THINKING WAS FLY THE PLANE TO THE GND FOR PRECAUTIONARY LNDG. AGAIN I LOOKED AT OIL PRESSURE AND TEMP; BOTH LOW BUT FINE. I APPEARED TO HAVE SOME RESERVE PWR WHEN NEEDED. I FLEW HANG GLIDERS FOR A COUPLE OF YRS AND JUST USED THAT SITE PICTURE TO LOCATE A SUITABLE SITE. LCL KNOWLEDGE TOLD ME THIS WAS FAR BETTER THAN XXX OR YYY. I SAW AN EXCELLENT OPPORTUNITY NNE OF A LAKE. I STATED I WAS GOING TO PUT IT DOWN IN A FIELD WITH AN OAK TREE IN THE CTR. I HAD RESERVED SOME PWR AND WITH A MENTAL SNAPSHOT OF THE FIELD I SWUNG DOWNWIND AND USED SOME PWR BRIEFLY TO PULL ME THROUGH A 160 DEG TURN. PULLING THE THROTTLE ALL THE WAY BACK I FEATHERED IN FOR A LNDG. I HAD TO USE SOME L RUDDER TO TURN AWAY FROM AN APCHING AG ROAD WHILE ON ROLLOUT AND CAME TO A STOP FACING 90 DEG L OF MY ORIGINAL APCH. I PULLED THE MIXTURE TO IDLE CUTOFF AND SHUT OFF THE MAGNETOS AND STROBE LEAVING THE RADIO AND XPONDER ON. I RADIOED MY LATITUDE LONGITUDE; CELL NUMBER AND NO DAMAGE TO PLANE PERSONS OR PROPERTY TO APCH. I SHUT OFF THE MAIN BREAKER AT XD17 I HOPPED OUT TO LOOK AT THE ENG; NO SMOKE; NO OIL SPLATTERS; NO WIRES DANGLING; NO HOLES IN THE ENG. I HAD 6.5 QUARTS OIL BROWNISH AND CLR; SUMPED FUEL UNDER THE ENG; CLR AND BLUE. STANDING BACK BOTH FUEL CAPS WERE ON. I REMOVED THE R WING GAS CAP; AND COULDN'T FEEL ANY GAS BUT HEARD SOME SLOSH IN THE TRAILING EDGE. I WALKED OVER TO THE L WING STANDING ON THE TIER 1 TAB OF THE CAP WAS ABOVE THE RIM OF THE FUEL NECK THE OTHER UNDERNEATH. SECURE BUT ASKEW. THE POH STATES THE CAPS ARE NON-VENTED TYPE AND MUST BE PROPERLY SECURED OR FUEL WILL SIPHON OUT AND FEED UNEVENLY DURING FLT. AFTER CLRING IT WITH THE SHERIFFS; FIRE DEPT AND AN FAA/NTSB PERSON ON A CELL I TOOK 10 GALS OF FUEL FROM THE LAND OWNER AND DID A FULL PREFLT. MINUS MY SISTER AND BAGS I RAN UP THE ENG TO TEMP; CHKED MAGNETOS; CARB HEAT AND WENT THROUGH MY SHORT FIELD TKOF CHKLIST. TOOK OFF CLBED AND CIRCLED TO APPROX 5500 FT. THE ENG WAS RUNNING FINE AND FLEW TO ZZZ. I BELIEVE MY FAILURE TO CORRECTLY SECURE THE GAS CAP LED TO IMPROPER GAS GAUGE READING AND POSITIVE AND NEGATIVE PRESSURES LEADING TO FUEL SURGES IN THE WINGS.