SMA ACFT ON INITIAL CLB. RPTR PLT FELT VIBRATION AND ROUGH RUNNING ENG AND REQUESTED AN IMMEDIATE RETURN TO ARPT. THOUGH RPTR DIDN'T DECLARE AN EMER; THE TWR DID FOR HER.

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TAKING OFF RWY 13L AT BFI AT 50 FT I NOTICED A SLIGHT VIBRATION. I LEANED THE MIXTURE SLIGHTLY AND IT SMOOTHED OUT. CONTINUING MY CLB AT 300 FT THE ENG BEGAN TO RUN ROUGH AND SPUTTER. CONTINUING TO CLB STRAIGHT AHEAD I CALLED THE TWR AND REQUESTED AN IMMEDIATE RETURN TO THE RWY. I DID NOT AT THIS TIME SPECIFY WHICH RWY BECAUSE I INTENDED TO CLB FOR AS LONG AS I COULD FIRST. PROBABLY FROM THE SOUND OF MY VOICE THE TWR TOLD A B767 ON FINAL FOR RWY 13R TO 'GAR FOR AN EMER IN THE AREA.' I DID NOT DECLARE AN EMER BUT I'M SURE THE TWR COULD HEAR THE FEAR IN MY VOICE. THE ENG CONTINUED TO PRODUCE SOME PWR AND AT APPROX 500 FT I BEGAN A L XWIND TURN TO AVOID THE B767 ON THE R AND TOLD THE TWR I WOULD LAND ON RWY 13L. THE TWR TOLD ME I WAS CLRED TO LAND ON ANY RWY (THEY SAID THIS TWICE) AND I RESPONDED THAT I WOULD LAND ON RWY 13R (THE LONG RWY). I LANDED WITHOUT INCIDENT (FURTHER) AND UPON REQUEST; I WAS CLRED TO TAXI TO PARKING; WHERE A POLICE OFFICER TOOK MY NAME AND ADDRESS. I THANKED THE TWR; THE FIRE DEPT AND THE POLICE FOR THEIR QUICK ACTION AND STATE OF READINESS. BECAUSE OF EVERYONE'S PROFESSIONALISM FURTHER PROBS DID NOT ARISE. EVERYONE IN MY OPINION DID WHAT THEY NEEDED TO DO.