FLT DIVERTED TO MCI WHEN 3000 LBS OF FUEL WAS LOST FROM THE R WING TANK.

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FLT DEPARTED STL TO SLC WITH STUDENT CAPT IN L SEAT OF MLG. RELEASE FUEL 38200 LBS (INCLUDING 20000 LBS IN CTR TANK). TKOF WITH ALL PUMPS ON PER COMPANY PROC. APCHING TOP OF CLB; FLT ATTENDANT ADVISED US THAT PAX IN REAR OF ACFT ON R SIDE OBSERVED FUEL STREAMING FROM OUTBOARD R WING. CAPT (INSTRUCTOR PLT) WENT BACK TO INVESTIGATE; RETURNED TO COCKPIT AND ANNOUNCED THAT WE WOULD LAND AT MCI DUE TO FUEL LOSS FROM R WING FUEL VENT. DECLARED EMER; LANDED MCI. PAX LATER RPTED FUEL LOSS STOPPED SHORTLY AFTER CAPT RETURNED TO COCKPIT. AFTER LNDG; FLT WAS CANCELLED; ACFT INSPECTED BY COMPANY MAINT PERSONNEL. FLEW SAME ACFT BACK TO STL NEXT MORNING ON REVENUE FLT. LOGBOOK SIGNOFF INDICATED NO PROB FOUND. LATER LEARNED THAT THIS PROB IS FAIRLY WELL KNOWN TO EXPERIENCED MLG PLTS; CAN BE CORRECTED BY TURNING OFF CTR PUMPS; ALLOWING MAINS TO BURN DOWN; THEN TURNING CTRS BACK ON. COMPANY FLT MANUAL HAS PROC FOR UNCOMMANDED FUEL TRANSFER; BUT DOES NOT MENTION LOSS OF FUEL FROM FUEL VENT. FUEL REMAINING AFTER LNDG MCI WAS APPROX 30000 LBS; INDICATING APPROX 3000 LBS OF FUEL WAS LOST OVERBOARD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: UNABLE TO CALL RPTR BUT A CALL TO ANOTHER MLG PLT REVEALED THAT; USUALLY; THE MOST FUEL LOST FROM A SIPHONING TANK IS ABOUT 3000 LBS.