PLT OF SMA PRIVATE ACFT EXPERIENCED A LOSS OF PWR AND WAS FORCED TO LAND.

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LEVELING AT 5500 FT -- 40 MIN INTO FLT WITH FULL TANKS 100 LOW LEAD. TEMP -08 DEGS C; SMOOTH AIR; BAROMETER 30.56; ENG CYCLED FROM 2300 RPM TO 1200 RPM REPEATEDLY TO LNDG ON PRIVATE GRAVEL STRIP (FREEDOM-MT WASHINGTON- GATEWAY). SUBSEQUENT RUNUP AND FLTS SHOW NO PROBLEM. 2 ATTENDING A&P MECHS STATED THEIR BELIEF AS VALVE PROBLEMS. VALVES HAD BEEN CHKED; PUSHROD CLEANED 16 HRS PRIOR. FUEL SYS PLACARDED 'SWITCH TO SINGLE TANK OP IMMEDIATELY REACHING CRUISE ALTS ABOVE 5000 FT.' HAD NOT LEVELED. REACHED 5500 FT AND HAD DECIDED TO IMMEDIATELY DSND TO LNDG AT KBML; WITH R TURN TO CLR MOUNTAINS. DENSITY ALT BELOW 4000 FT ALWAYS. ALL MECHS BELIEVE PLACARDED PROBLEM DID NOT OCCUR. 2 PREVIOUS OCCURRENCES (ACFT). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THERE HAS BEEN NOTHING FOUND TO CAUSE THE ENG FAILURE. HOWEVER; WATER IN THE FUEL SYS WAS DISCUSSED AND A FAULTY IGNITION SYS. HE SAID THAT THE LATTER WAS A POSSIBILITY SINCE THE MECH DID FIND A PROBLEM WITH ONE OF THE MAGNETOS. HE FURTHER STATED THAT THE FLYING CLUB IN WHICH THIS ACFT IS USED; IS ADDING NEW PROCS TO WRITE UP MECHANICAL DISCREPANCIES FOR NOT ONLY THE MAINT PERSONNEL; BUT THE OTHER CLUB MEMBERS SO THAT THE ACFT WILL NOT BE USED WITH A KNOWN PROBLEM.