HOME BUILT ACFT SUFFERS EXTREME ENG VIBRATION. PLT GETS VECTOR TO NEAREST ARPT FROM JACKSON APCH CTLR. POSTFLT INSPECTION AND TEST FLT FAILED TO REVEAL FURTHER SYMPTOMS. A CASE OF POSSIBLE FUEL CONTAMINATION FROM REFUELING AT F89 ARPT; WINNSBORO; LA.

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I FLEW FROM TCL TO F85 WITHOUT A PROB. SEVERAL HRS LATER (AFTER REFUELING AND DOING A PREFLT) I DEPARTED F89 ENRTE TO TCL. I WAS FLYING MY 1989 RV3A. SOME 25-30 MINS INTO THE FLT ZME INSTRUCTED ME TO CONTACT JACKSON; MS; APCH. I DID; AND THEY ACKNOWLEDGED. I WAS AT 5500 FT IN RELATIVELY SMOOTH AIR. GOOD VISIBILITY AND ALL GAUGES IN THE GREEN. WITHOUT WARNING; I EXPERIENCED SEVERE ENG ROUGHNESS. I COULDN'T READ THE GAUGES AND THE AIRPLANE WAS BUCKING SIGNIFICANTLY. WITHIN SECONDS I: 1) CLOSED THE THROTTLE AND 2) TOLD APCH THAT I HAD RATHER SEVERE ENG ROUGHNESS AND NEEDED TO MAKE A PRECAUTIONARY LNDG AT THE NEAREST AVAILABLE ARPT. THEY TOLD ME RAYMOND WAS AT MY 1 OR 2 O'CLOCK AND ABOUT 10 MI. THEY ASKED IF I NEEDED VECTORS. I RPTED THAT I WAS GOING TO RAYMOND AND THAT I WOULD NEED VECTORS SINCE I COULDN'T READ MY VERT COMPASS WHICH WAS VIRTUALLY SPINNING (NO GYROS AVAILABLE). THEY GAVE VECTORS AND I RPTED RAYMOND IN SIGHT. THEY WERE TALKING WITH RAYMOND AND TOLD ME THERE WAS NO TFC. THEY TOLD ME I COULD LAND STRAIGHT IN BUT BECAUSE I WAS IN A TAILDRAGGER AND HAD SUFFICIENT ALT; I ELECTED TO DO A DOWNWIND AND LAND INTO THE WIND. I ASKED FOR THE ARPT FREQ (UNICOM); BUT APCH TOLD ME TO STAY WITH THEM. I LANDED WITHOUT INCIDENT. FIRE AND POLICE ARRIVED WITHIN MINS. BY THE TIME I LANDED; THE ENG WAS IDLING SMOOTHLY. WE CHKED FUEL; ENG AND COULDN'T FIND ANYTHING. RUN-UPS AND TEST FLTS DID NOT DUPLICATE THE PROB. I DID NOT DECLARE AN EMER AND I REMAINED ON APCH FREQ PER THEIR REQUEST. I FLEW THE PATTERN DUE TO WIND CONDITIONS. I BELIEVE EVERYONE ACTED APPROPRIATELY UNDER THE CIRCUMSTANCES. APCH WAS A REAL HELP.