AN SMA PLT HAD AN ENG STOPPAGE DUE TO AN INSECT BLOCKING THE FUEL TANK VENT LINE.

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DEPARTED SCK ENRTE TO IZA ON MM/DD/92 AT HH35 HRS LCL AFTER STANDARD PREFLT/RUNUP PER CHKLIST (ENCLOSED). FUEL AT TIME OF DEP WAS 56 GALLONS; OF WHICH 40 WAS IN TIP TANKS. FUEL CONSUMPTION FOR THIS ACFT IS 12 GALLONS PER HR. ESTIMATED TIME ENRTE WAS 2 HRS. CLBED TO CRUISE ALT OF 5500 FT MSL; LEVELED OFF; TURNED ON BOOST PUMP ON LOW; SWITCHED FROM MAIN TANK (REQUIRED FOR TKOF AND LNDG) TO R TIP TANK; AND AFTER 30 SECONDS; TURNED OFF BOOST PUMP. ENG LOST PWR ABOUT 1 1/2 MINS (ESTIMATED) AFTER CHANGING TANKS. FUEL ON BOARD AT THIS TIME WAS ESTIMATED TO BE 52 GALLONS; CONSISTING OF 12 GALLONS IN MAIN TANK; AND 40 GALLONS IN TIPS. ESTABLISHED GLIDE TO NEAREST ARPT (NALF CROWS LNDG) AND COMMENCED RESTART PROC CONSISTING OF RETURN TO MAIN TANK FUEL; TURN ON BOOST PUMP; MIXTURE RICH; FULL THROTTLE; AND DECLARED EMER. ENG RESTARTED 500 FT AGL ON SHORT FINAL TO CROWS LNDG; WHICH TWR WAS CURRENTLY CLOSED. DURING THIS TIME; WE WERE UNDER FLT FOLLOWING WITH CASTLE APCH; HAVING BEEN HANDED OFF FROM SCK TWR PRIOR TO THE EVENT. UPON RESTART; NOTIFIED CASTLE WE WOULD PROCEED AT 2500 MSL TO MOD FOR LNDG. ELECTED TO UTILIZE MAIN TANK (NOW ABOUT 10 GALLONS) SINCE PROBLEM WITH TIP TANK WAS UNDIAGNOSED; AND DIDN'T WANT TO REPEAT PROBLEM. UPON TRANSFER TO MOD TWR; ADVISED THEM OF LOW FUEL AND REQUESTED AND RECEIVED PERMISSION FOR 2500 FT XWIND APCH TO REMAIN CAPABLE OF GLIDE TO RWY IN THE EVENT OF REPEAT OF PROBLEM. LANDED WITHOUT INCIDENT; WITH FULL PWR AVAILABLE. FUEL ON BOARD AT LNDG WAS 44 GALLONS; OF WHICH 4 GALLONS WERE IN THE MAIN TANK. CAUSE OF ENG OUT WAS DETERMINED BY MECH A OF FBO AT MOD ARPT TO BE 'LEAF ROLLER' (FLYING INSECT) DEBRIS PACKED INTO R TIP TANK VENT TUBE; TOTALLY OBSTRUCTING AIR FLOW IN THE VENT. TANK VENTS ARE APPROX 1/8 INCH TUBING; WHICH ARE OPEN TO AIR AT A POINT UNDER THE WING ATTACHMENT POINT. THERE ARE NO SCREENS ON THE VENT OPENINGS. THE VENT WAS CLRED; AND THE L VENT CHKED AND ALSO CLRED OF SIMILAR DEBRIS (ALTHOUGH NOT TOTALLY CLOSED); AND THE ACFT WAS RETURNED TO SVC. SUBSEQUENT TESTS ON MM/DD/92 INDICATED THAT WITH A FULL TIP TANK; THE ENG WOULD OPERATE WITHOUT LOSS OF PWR AT RUNUP SPD FOR A PERIOD IN EXCESS OF 3 MINS WITH A FULLY CLOGGED VENT; THUS MAKING A NORMAL 60 SECOND CHK MEANINGLESS TO DISCOVER THE VENT OBSTRUCTION. THE PREFLT WILL NO LONGER RELY ON AN OPERATING TEST OF THE FUEL SELECTOR AS THE SOLE DETERMINANT OF THE CONDITION OF THE FUEL TANK VENT SYS. ALTHOUGH A SEASONED MECH HELD THE OPINION THAT GAS FUMES DISCOURAGED INSECT NESTING IN VENT TUBES; THIS OCCURRENCE SUGGESTS OTHERWISE. A PHYSICAL CHK OF EACH VENT IS NOW PART OF THE PREFLT CHKLIST.