POTENTIAL CONFLICT WITH AN SMA WHEN ENGINE BEGAN RUNNING ROUGH.

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PRIOR TO THIS DEP HAD NOT EXPERIENCED ANY PROBS WITH THE ENG. CHKED THE FUEL TANKS PRIOR TO ENG START AND NOTED SLIGHTLY MORE THAN HALF TANKS; WHICH WAS WHAT I EXPECTED. NORMAL ENG START AND RUNUP. DURING TAXI I NOTICED A SMA IN THE PATTERN MAKING FULL STOPS ON RWY 31L. WAS CLRED FOR RIGHT DOWNWIND DEP ON RWY 31R AND NOTICED SMA TURNING HIGH; CLOSE IN BASE TO FINAL FOR RWY 31L. SINCE ACFT APPEARED TO BE HEADED FOR RWY 31R; I CONFIRMED HIS ASSIGNED RWY WITH TWR. THEY STATED HE WAS LNDG RWY 31L AND WHEN I SAW HIM CORRECT FOR OVERSHOOT; I COMMENCED THE TKOF ROLL ON RWY 31R. DURING THE TKOF; SMA REQUESTED TO BE CLRED FOR THE 'OPTION' AND THEN COMMENCED A GO AROUND WITH TWR APPROVAL. HOWEVER; DUE TO THE NOISE AND ACTIVITY OF THE TKOF; I DID NOT HEAR ANY OF THIS AND ASSUMED THE SMA HAD LANDED; FULL STOP. AT APPROX 500' MSL (400' AGL) THE ENG BEGAN TO RUN ROUGH FOLLOWED BY A LARGE DROP IN RPM AND MANIFOLD PRESSURE. I LOWERED THE NOSE AND HEADED FOR AN OPEN PARK AREA TO MY LEFT AND TOLD TWR I HAD AN ENG PROB. AS SOON AS I BEGAN THE TURN; MY FATHER CALLED THE SMA SLIGHTLY BELOW AND ABOUT 600' BEHIND US ON HIS GO AROUND. HE SAW US TURN INTO HIS PATH AND BEGAN AN IMMEDIATE TURN TO DOWNWIND; WITH TWR APPROVAL. MEANWHILE; I REGAINED FULL PWR (FOLLOWING THE SURGE) FOR ABOUT 5 SECS AND WAS TALKING TO THE TWR WHEN THE ENG COUGHED A SECOND TIME AND BEGAN TO QUIT. I SWITCHED TANKS; TURNED ON THE ELECTRIC BOOST PUMPS AND AGAIN BEGAN TO SET UP FOR AN EMER LNDG. THE ENG RECOVERED; I CLBED TO 1100' MSL AND MADE A NORMAL APCH AND LNDG ON RWY 31L. GND ASKED ME TO CALL THEM AFTER SHUTDOWN BUT I SAID I WOULD RATHER COME TO THE TWR TO DISCUSS WHAT HAD HAPPENED. THEIR PRIMARY CONCERN WAS THE SEP BTWN ACFT AND THEY COULDN'T SEE HOW CLOSE WE REALLY WERE DUE TO THE ANGLE AND DISTANCE FROM THE TWR. ONCE THE SMA BEGAN HIS TURN; HE NEVER WAS A FACTOR AND I DON'T THINK WE WOULD HAVE COLLIDED EVEN IF HE HADN'T SEEN US TURN. BUT I WAS STARTLED TO SEE HIM THERE AND WOULD NOT HAVE TURNED LEFT HAD I KNOWN HE HAD BEGUN A MISSED APCH. EITHER WAY; IT WAS A TOUGH CHOICE AS THE RESIDENTIAL AREA OFF THE DEP END OF RWY 31L OR 31R IS HVY AND I MOST LIKELY WOULD HAVE COME TO REST IN A YARD HAD I GONE STRAIGHT AHEAD. THE FIELD I WAS HDG FOR WAS ABOUT 30-40 DEGS TO MY LEFT AND WITHIN RANGE. THE CTLR WHO WAS WORKING LCL SAID THIS WAS THE FIRST MISSED APCH THE SMA HAD MADE SINCE ENTERING THE PATTERN AND THEY DIDN'T TELL ME BECAUSE THEY KNOW A PLT IS BUSY DURING THE TKOF ROLL AND CLBOUT AND DON'T LIKE TO BOTHER HIM WITH TFC CALLS THEN. THEY SAID THEY COULD HEAR THE ENG MISSING IN THE BACKGROUND AS I WAS TALKING TO THEM; SO THEY CLRED THE AREA FOR ME TO LAND ON RWY 13L OR R IF I WANTED TO ATTEMPT A 180 DEG TURN. AS SOON AS I FELT PWR RESTORED I TOLD THEM I WOULD LAND USING THE NORMAL PATTERN. THE NEXT 2 DAYS WERE SPENT DRAINING FUEL TANKS AND SUMPS; CLEANING FUEL FILTERS; BOWLS AND STRAINERS AND CHKING THE VENT SYS FOR THE CAUSE OF THE ENG MISS. WE HAVE FOUND NOTHING THAT INDICATES A FUEL CONTAMINATION PROB OR AIR FILTER OR FUEL VENT SYS PROB AND DISCUSSIONS WITH SEVERAL MECHS AND INSPECTORS HAVE LED US TO SUSPECT A FUEL VAPOR LOCK PROB. ANOTHER PLT REVEALED A SIMILAR TYPE MISS WHILE AT CRUISE IN AN OLDER SMA LIKE MINE DURING A HOT DAY. OAT WAS 85 DEGS AT THE TIME I WAS FLYING BUT THE RUN UP WAS SMOOTH AND I HADN'T SWITCHED TANKS FOR THE PREVIOUS 50 MINS OF FLYING. NORMAL PROC DOESN'T CALL FOR THE AUX FUEL PUMP UNLESS THE ENG DRIVEN PUMP FAILS SO I WAS DOING IT BY THE BOOK. I HAVE LEARNED TO KEEP THE EARS TUNED AS MUCH AS POSSIBLE; EVEN DURING THE 'BUSY' TIMES; BECAUSE THAT APPEARS TO BE WHEN MURPHY'S LAW IS WORKING ITS HARDEST...