PLT OF LJ35 VENTED FUEL ONTO RAMP AT BFI WHEN HE GOT DISTR FROM THE FUEL XFER PROC HE INITIATED TO CORRECT AN OUT OF BAL CONDITION.

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WHILE PREPARING THE ACFT; LR35A; FOR DEP AT BFI; I SWITCHED THE XFLOW TO OPEN AND STARTED THE R FUEL PUMP FOR XFER ABOUT 200 LBS OF FUEL TO THE L WING TO BAL THE FUEL LOAD. WHILE THE FUEL WAS XFERRING; I OBTAINED OUR OUTBOUND IFR CLRNC TO ICT AND PROCEEDED TO PROGRAM THE FLT PLAN INTO THE FMS. I THEN WENT INTO THE FBO FACILITY AT BFI TO USE THE RESTROOM AND GREET MY PAX. WHEN I LOOKED OUT AT THE ACFT ABOUT 7 MINS LATER; THERE WAS FUEL VENTING OUT OF THE L TIP TANK. IMMEDIATELY; I REALIZED WHAT I'D DONE AND WENT STRAIGHT TO THE ACFT AND SWITCHED OFF THE R PUMP AND TURNED THE L PUMP ON. THE VENTING FUEL STOPPED AND I REMAINED WITH THE ACFT UNTIL THE FUEL SYS WAS BALANCED. THE TOTAL FUEL SPILLED ONTO THE FUEL RAMP TOTALED ABOUT 3-4 GALS. THE L TIP TANK HAD BECOME FULL AND AS THE PUMP CONTINUED TO RUN WAS VENTING FUEL OVER-BOARD. SOME POINTS ABOUT THE FUEL SYS ARE THAT THERE IS NO VISUAL OR AUDIO REMINDER THAT FUEL XFER OPS ARE UNDERWAY. WHEN I GOT BUSY WITH OTHER PREPARATIONS; I FORGOT THAT I WAS XFERRING FUEL. THE ONLY WAY TO KNOW THAT FUEL IS MOVING IS TO CHK THE SWITCHES AND MONITOR THE FUEL GAUGES CONTINUOUSLY. BY FORGETTING THAT FUEL WAS XFERRING; I CREATED AN IMBAL IN THE FUEL LOAD AND SPILLED ABOUT 3-4 GALS. THESE PROBS WERE EASILY CORRECTED AND CREATED NO PROB OR DELAY IN OUR DEP. I THEN LEARNED THAT WHEN MAKING ADJUSTMENTS TO THE LEAR 35 FUEL SYS; IT DEMANDS CONTINUOUS ATTN. I WILL DO MY BEST TO NEVER LET THIS OCCUR AGAIN. AS A CAPT; I'LL ALSO WARN OTHERS OF THIS DANGER. IN MY 4 YRS AS A CAPT ON THE LEAR; THIS IS MY FIRST MISTAKE WITH THE FUEL SYS AND I DO NOT INTEND TO REPEAT THIS AGAIN.