GA PLT FELT ILL AT CRUISE ALT AND DESCENDED FROM 12000' TO 8000 BEFORE HE WAS ABLE TO CONTACT ARTCC.

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FLYING LEVEL AT 12000' I BECAME LIGHT HEADED AND CALLED CENTER 3 TIMES ON THE CURRENT FREQ. I COULD NOT REACH THEM AND FELT IT WAS NECESSARY TO IMMEDIATELY DSND. I THEN DSNDED AND CALLED THEM ON A PREVIOUS FREQ; 132.5 (MEM). I REACHED THEM THIS TIME; EXPLAINED MY DILEMMA AND WAS CLRED TO 8000'. I CONTINUED TO FEEL ILL AND REQUESTED AN IMMEDIATE LNDG AT WEST MEMPHIS; AR. ZME APCH WAS THEN VERY HELPFUL AND I LANDED W/O INCIDENCE. I HAD THE PLANE CHKED OUT BOTH THERE AND IN ST LOUIS AND CARBON MONOXIDE COULD HAVE BEEN THE PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: MAINT FOUND A FAULTY PRESSURIZATION VALVE THAT COULD HAVE BEEN LEAKING CO INTO THE CABIN. SEVERAL EXHAUST FITTINGS WERE TIGHTENED AND THE ACFT WAS FLOWN TO HOME BASE UNPRESSURIZED WITH NO ILL EFFECTS TO THE PLT. WHEN CONTACT WAS MADE WITH ARTCC THEY WERE VERY HELPFUL ASKING RPTR IF HIS FINGERS WERE BLUE AND SUGGESTING HE OPEN HIS COCKPIT WINDOWS. RPTR SAYS HE IS NOT HESITANT ABOUT DECLARING AN EMER BUT DIDN'T EVER THINK OF USING HIS TRANSPONDER FOR THIS. RPTR HAS BEEN FLYING FOR 20 YRS BUT THIS ACFT AND A TRANSPONDER IS NEW TO HIM AND HE IS STILL LEARNING TO USE ALL THE NEW EQUIP.