PLT OF C150 HAS LOSS OF ENG PWR JUST AFTER TKOF AND MAKES AN OFF ARPT LNDG WITH NO DAMAGE.

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I WAS PIC (AND ONLY PERSON ON BOARD) OF C150; DEPARTING POPE FIELD (GFD). I EXPERIENCED A SEVERE PWR LOSS AT FULL THROTTLE WHILE XING THE N THRESHOLD OF RWY 36. MY ALT WAS APPROX 250 FT AGL. I REDUCED PWR SETTING AND ENG SEEMED TO RUN NORMALLY; BUT PRODUCING LESS THAN FULL PWR. I INCREASED PWR SETTING; BUT ENG ONLY INCREASED SLIGHTLY IN PWR. HIGHEST PWR SETTING WAS NOT SUFFICIENT TO SUSTAIN LEVEL FLT AND ENG PWR WAS CONTINUING TO DECREASE AT FULL THROTTLE. I ESTABLISHED BEST GLIDE SPD OF 70 MPH AND PICKED A FIELD OF SUFFICIENT LENGTH FOR SAFE LNDG TO MY L (W). I PERFORMED A 90 DEG L TURN AND BEGAN A FINAL APCH FOR LNDG IMMEDIATELY STRAIGHT AHEAD. I REDUCED PWR TO IDLE AND APPLIED FULL FLAPS; DSNDED; FLARED; AND LANDED AS I NORMALLY WOULD; APPLIED BRAKES UNTIL STOPPED. I OBSERVED THAT ENG WAS STILL RUNNING AT A NORMAL IDLE SPD; AND FOLLOWED NORMAL SHUT DOWN PROC. I THEN EXITED THE ACFT AND PROCEEDED TO CHK FUEL LEVELS BY VISUAL INSPECTION. UPON OPENING THE L TANK CAP; A RUSHING AIR SOUND WAS OBSERVED UNTIL THE CAP WAS COMPLETELY REMOVED. IT FELT AS THOUGH A SUCTION FORCE WAS RETAINING THE CAP AT FIRST. NO SUCH SOUND OR OBSERVATION WAS NOTICED UPON REMOVAL OF THE R CAP. FUEL QUANTITY IN BOTH TANKS WAS VISUALLY OBSERVED TO BE APPROX 1.5 - 2.0 INCHES (3-4 GALLONS PER TANK; 6-8 GALLONS TOTAL). PLT AND ACFT WERE UNDAMAGED. PLT'S CONCLUSION: THE FUEL VENTING SYS WAS PROBABLY BLOCKED (POSSIBLE INSECT INFESTATION). OWNER/PLT WILL HAVE FUEL VENT SYS INSPECTED PRIOR TO FURTHER OP OF ACFT. THIS ACFT HAS BEEN FLOWN BY SAME PLT FOR 40 HRS ON A REGULAR BASIS SINCE PURCHASE DATE; 45 DAYS FROM THE DATE OF THIS INCIDENT. NO PROBS HAVE EVER BEEN OBSERVED WITH THIS ACFT DURING ANY OP THE PREVIOUS 40 HRS OF FLT BY THIS OWNER/PLT. PREVENTIVE MEASURES: NONE. FUEL VENT WAS INSPECTED DURING PREFLT WITH NO ABNORMALITIES OBSERVED. IF INSECT INFESTATION (OR OTHER BLOCKAGE) IS BEYOND A POINT OF VISUAL DETECTION; THEN THERE IS NO WAY TO DETERMINE THIS PROB EXISTS PRIOR TO FLT; DURING A NORMAL PREFLT INSPECTION. BLOWING ON THE VENT TUBE IS PROBABLY UNDESIRABLE AS THIS WOULD EITHER FORCE THE OBSTRUCTION FURTHER INTO THE VENT OR ALL THE WAY INTO THE TANK; WHERE IT COULD CAUSE THE SAME OR OTHER PROBS LATER. SOLUTION: HAVE A LICENSED A&P MECH INSPECT AND TEST SYS IN ORDER TO DETERMINE EXACT CAUSE OF THIS PROB.