AN SMT PLT EXPERIENCES A LOSS OF PWR ON THE L ENG AND DECIDES TO DIVERT TO AN ENRTE ARPT. THE INTAKE VALVE IN THE #2 CYLINDER ON THE L ENG WAS FOUND TO BE DAMAGED.

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WE WERE AT 13000 FT ON AN IFR FLT PLAN FROM EAU TO VAY. WE HAD BEEN CLRED DIRECT ERIE. S OF FLINT; MI; THE L ENG BEGAN TO EXPERIENCE A FLUCTUATION IN FUEL FLOW. WITHIN MINS; THE L ENG BEGAN TO FLUCTUATE BTWN 2100 AND 2500 RPM'S (CRUISE SETTING). I FIRST ADVISED ATC OF A ROUGH RUNNING ENG. THEY WERE VERY HELPFUL. I THEN DECIDED TO RETARD THE L ENG TO ZERO THRUST (12 INCHES MANIFOLD PRESSURE) BECAUSE I HAD GOOD OIL TEMP AND PRESSURE ON THE L ENG. ATC CLRED ME TO 12000 FT AND ADVISED THAT SELFRIDGE WAS 12 O'CLOCK AND 5 MI. I REQUESTED A DIFFERENT ARPT. ATC ADVISED DETROIT METRO WAS 40 MI; BUT DETROIT CITY WAS 8 MI. I DSNDED TO 12000 FT; BUT MAY HAVE GONE LOWER WITHOUT ATC CLRNC. I WAS EVENTUALLY CLRED TO THE FIELD. ATC WAS VERY HELPFUL. I LANDED WITHOUT FURTHER INCIDENT. NO DAMAGE TO THE ACFT EXCEPT AN INTAKE VALVE ON THE #2 CYLINDER ON THE L ENG. THE ENG HAD GIVEN ME DIFFICULTY IN STARTING ON THE LAST 3 FLTS. AFTER EACH START; I WOULD DO MULTIPLE RUN-UPS AND WOULD RUN THE ENG AT IDLE WITHOUT A PROB. I SHOULD'VE HAD A MECH THOROUGHLY INSPECT THE ENG REGARDLESS.