ACFT EQUIP PROB. ROUGH RUNNING ENG.

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ON JUN/SUN/95; I FILED ON IFR FLT PLAN FROM IDI (INDIANA COUNTY/JIMMY STEWART ARPT) IN PA TO BLM; ALLAIRE ARPT; NJ. WX WAS CAVOK WITH A LIGHT W BREEZE. I DEPARTED RWY 28 AND TURNED E TOWARD TYRONE (TON) VOR. CLBING THROUGH 3000 FT I NOTED A MILD BUFFETING. I CHKED THE FLAPS UP; ENG INSTS; ETC. EVERYTHING APPEARED NORMAL. I ELECTED TO RETURN TO IDI TO CHK THE EXTERIOR OF THE ACFT AND ZOB GAVE ME A HDG BACK TO THE FIELD. AT THE TIME I WAS 3-4 MI E OF IDI. AS I SLOWED AND PULLED THE PWR BACK THE BUFFETING CEASED AND EVERYTHING SEEMED NORMAL. AT THIS JUNCTURE I ALMOST OPTED TO CONTINUE THE FLT TO BLM BUT THOUGHT BETTER OF IT. I TOLD ZOB THAT IDI WAS IN SIGHT AND TO CANCEL MY IFR FLT PLAN; WHICH THEY DID. LNDG AND TAXI IN WAS NORMAL. I ACCOMPLISHED ANOTHER MAGNETO CHK; 'EXERCISED' THE PROP; CHKED AND RE-CHKED THE INSTS; PRESSURES; VACUUM; ETC. ALL INDICATIONS WERE NORMAL. UPON PARKING I ACCOMPLISHED A METICULOUS WALK-AROUND OF THE ACFT. NOTHING UNTOWARD WAS DISCOVERED. I WAS ABOUT TO ADVISE MY PAX THAT I WAS GOING TO CANCEL OUR TRIP HOME WHEN ONE OF THEM INFORMED ME THAT ON CLB OUT HE NOTICED THAT THE TOP R SECTION OF THE REAR PAX DOOR WAS SLIGHTLY AJAR AND NOT FLUSH WITH THE FUSELAGE. I DIDN'T SEE HOW THIS WAS POSSIBLE SINCE I HAD PERSONALLY LOCKED AND CHKED THE DOOR AND ON TAXI OUT HAD ASKED MY PAX TO RE-CHK THE TOP LATCH CLOSED; WHICH HE CONFIMRED. NEVERTHELESS; I ATTRIBUTED THE MILD BUFFET WITH THE DOOR NOT BEING 'TRUE' WITH THE SKIN OF THE ACFT. TO SATISFY MYSELF; I ACCOMPLISHED ANOTHER ENG RUN-UP; TOOK OFF SOLO; AND FLEW AROUND THE PATTERN AT AN EXAGERATED AIRSPD AND PWR SETTINGS. THE CRAFT HANDLED SMOOTHLY; INDICATIONS WERE GOOD; AND 'SHE PURRED LIKE A KITTEN.' FEELING SATISFIED THAT THE PROB WAS THE REAR DOOR AND THAT THE PLANE WAS AIRWORTHY; I AGAIN FILED AN IFR FLT PLAN TO BLM. YET ANOTHER MAGNETO CHK AND ENG RUN-UP SHOWED NOTHING AMISS; AND I DEPARTED RWY 28 WITH PAX IN TOW. 5 SECONDS BEFORE LIFT OFF I ASKED MY PAX TO RE-CHK THE INTEGRITY OF HIS DOOR AND HE SAID IT WAS FULLY CLOSED. TKOF; FLAP RETRACTION; AND INITIAL CLB OUT WERE NORMAL. AT APPROX 700 FT AGL I COMMENCED A L BANK TO THE E. AFTER ABOUT 90 DEGS OF TURN THE ACFT BEGAN TO EXHIBIT A MORE PRONOUNCED VIBRATION THAN IT HAD ON THE FIRST LEG. THE FUEL BOOST PUMP WAS STILL ON AND I SWITCHED TANKS; TO NO AVAIL. I DECREASED MY ANGLE OF BANK AND MAINTAINED 700 FT AGL; ALL THE WHILE HDG BACK TO IDI. I ANNOUNCED MY INTENTIONS ON CTAF AND LANDED ON RWY 10. AN A AND P MECH ON THE FIELD AT THE TIME LOOKED AT THE ENG AND INITIALLY TOLD ME THE PLANE HAD A PARTIALLY BLOCKED FUEL INJECTOR. UPON FURTHER INSPECTION THE NEXT DAY; JUN XX; HE ALSO INFORMED ME THAT THE #2 CYLINDER HAD A COMPRESSION OF 15/80; 80/80 BEING NORMAL. I'M NOT A MECH BUT I COULDN'T UNDERSTAND; AND STILL DON'T; WHY THE AIRPLANE BEHAVED FLAWLESSLY WHEN I FLEW IT BY MYSELF. WHY DIDN'T IT DISPLAY SOME PROB WHEN I WAS IN THE PATTERN? WITH THE ADVANTAGE OF HINDSIGHT AND MY FEET PLANTED FIRMLY ON THE GND; I STILL DON'T THINK I WOULD HAVE DONE ANYTHING DIFFERENTLY. MOST PLTS; AFTER HAVING A 'PROB;' SAY THEY SHOULD HAVE DONE THIS OR DONE THAT; NEVER AGAIN; ETC. SUCH IS NOT THE CASE WITH ME. I'M AN EXPERIENCED 11000 HR ATP; WITH 30 YRS OF FLYING EXPERIENCE. WITH THE FACTS I HAD AT MY FINGERTIPS; I BELIEVE I WOULD HAVE MADE THE SAME DECISION. THANKFULLY; THINGS DID TURN OUT WELL AND WE DID RENT A CAR AND DRIVE TO OUR DEST. DID SOME OTHER PLT OR PLTS NOTE A 'GLITCH' OR A 'FUNNY SOUND' AND NOT BOTHER TO WRITE IT UP; OR DID A MECH ON AN OIL CHANGE OBSERVE SOMETHING BUT FIGURED IT COULD WAIT FOR THE UPCOMING ANNUAL? DID THIS BLOCKED FUEL INJECTOR AND POOR COMPRESSION HAPPEN ALL AT ONCE; OR WAS IT SO INSIDIOUS AND BENIGN SO AS NOT TO BE WORTH MENTIONING? THIS INCIDENT DEMONSTRATES THAT EVEN A SCRUPULOUS; EXACTING PLT WHO TRIES TO COVER ALL BASES AND STAY AHEAD OF THE AIRPLANE AND DEVELOPING SIT CAN STILL GET BURNED; ALMOST. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: FIRST TKOF RPTR EXPERIENCED MILD BUFFETING; WHICH WAS ATTRIBUTED TO THE REAR PAX DOOR BEING SLIGHTLY AJAR. ALL OTHER INSTS AND INDICATIONS WERE NORMAL. AFTER INSURING THE REAR DOOR WAS SECURED; TEST RUN AND SOLO CHK OUT IN THE PATTERN; RPTR LANDED; PICKED UP PAX AND AGAIN DEPARTED. THIS TIME AFTER TKOF AND IN A L BANK ACFT EXHIBITED A MORE PRONOUNCED VIBRATION THAT IT HAD BEFORE; SO AGAIN RPTR RETURNED TO LAND. MECH AND RPTR DID AN ENG RUN-UP WITH NO ABNORMAL INDICATIONS. LATER MECH FOUND PARTIALLY BLOCKED FUEL INJECTOR AND LOW COMPRESSION IN THE #2 CYLINDER. RPTR HAD NO FURTHER FEEDBACK ON THE ENG.