ACFT EQUIP PROB -- SMA ENG RUNS ROUGH AS THOUGH APCHING FUEL STARVATION. LATER; AN INSECT NEST IS FOUND IN THE FUEL VENT SYS.

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DSNDING THROUGH 8000 FT RETURNING FROM SCOTT VALLEY (FT JONES) TO SAN CARLOS. I NOTICED WHAT SEEMED TO BE A ROUGH ENG OVER THE N SHORE OF CLEAR LAKE. I IMMEDIATELY TURNED ON THE FUEL PUMP AND ENRICHED THE MIXTURE; THEN CHKED THE ENG GAUGES. THE FUEL PUMP GAUGES INDICATED 10 GALS IN THE R TANK AND LESS THAN 5 GALS IN THE L. BEFORE LEAVING SCOTT VALLEY THE TANKS WERE VISUALLY INSPECTED AND APPROX HALF TANKS WERE CONFIRMED. I THEN CALLED ZOA AND REQUESTED AN ARPT VECTOR. THEY GAVE ME A HDG. I WAS 8 MI FROM CLEAR LAKE; LAMPSON FIELD. I REQUESTED PRIORITY LNDG OVER THE UNICOM FREQ. UPON LNDG THE PROP STOPPED AND I HAD JUST ENOUGH ENERGY TO CLR THE RWY. I HAD HELP PULLING THE AIRPLANE IN AND FUELED IT UP. I THEN CALLED THE FAA AT THE MENDOCINO FSS TO MAKE SURE THAT I HAD LANDED AND WAS ALRIGHT. I DIDN'T DECLARE AN EMER AND THUS A RPT WAS NOT REQUIRED; AS I WAS TOLD BY THE FAA. I TOPPED OFF THE ACFT AND WAS ONLY ABLE TO FILL 42 GALS IN THE TANKS WHICH TOTAL 50 GALS. THE QUESTION IS HOW THE 8 GALS OF FUEL NEVER REACHED THE ENG. PRIOR TO START-UP I CHKED THE FUEL SYS AND DID A NORMAL PREFLT AND RUN-UP. THE ACFT OPERATED NORMALLY IN THE RUN-UP AND SOUNDED NORMAL. THE FLT BACK TO SQL WAS UNEVENTFUL AND SMOOTH. PRIOR TO LEAVING SQL I TOPPED OFF AGAIN BEFORE HDG BACK TO MRY. I RPTED THIS INCIDENT TO THE FLYING CLUB MGR AND MAINT DIRECTOR AND THEN GNDED THE AIRPLANE. THEY CHKED IT OUT THE NEXT DAY AND FOUND NOTHING WRONG WITH IT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFO: THE PLT LATER FOUND THAT THE FLYING CLB MECHS HAD DISCOVERED THAT AN INSECT NEST HAD CLOGGED THE FUEL TANK VENT. THIS PROBABLY CAUSED THE FUEL TO BE LESS THAN FREE FLOWING SINCE THE ENG PUMP WOULD BE WORKING AGAINST A VACUUM. THE PLT ALSO TALKED TO SEVERAL OTHER MEMBERS OF THE AERO CLUB WHO HAD LESSER; BUT SIMILAR EXPERIENCES WITH THE SAME BEECH MENTOR (T-34). SINCE THE INSECT NEST DISCOVERY NO FURTHER FUEL PROBS HAVE OCCURRED WITH THE ACFT.