INTREPID PLT OF M20P ON A LONG IFR XCOUNTRY ENCOUNTERS FREEZING MOISTURE WHICH CAUSES A PARTIAL LOSS OF PWR AND A DIVERSION TO A SAFE LNDG.

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I DEPARTED LOS ALAMOS; NM; DESTINED FOR SHAWNEE; OK. I CALLED FSS TO RECEIVE A SECOND BRIEFING 1 HR BEFORE FLT TIME. THE INFO WAS GIVEN THAT THE TOPS OF THE CEILINGS WERE TO BE 10K TO 24K DEPENDING UPON LOCATIONS BTWN MY DEP POINT AND DEST. AFTER REQUESTING OPTIONS FROM FSS; IT WAS SUGGESTED THAT I CONSIDER FLYING TO ROSWELL; NM; THEN TO WICHITA FALLS AND THEN TO SHAWNEE; OK; AS THIS RTE MAY ALLOW ME TO KEEP ABOVE THE CLOUD LAYERS. I OPENED MY IFR FLT PLAN OVER SANTA FE IN VFR CONDITIONS 1 HR BEFORE DARK AND CLBED TO AN ALT WHICH WAS ABOVE THE CLOUDS. I REQUESTED AND STAYED WITH 11K BLOCKED TO 13K. ENRTE TO ROSWELL; I NOTICED THE CLOUDS WERE WELL BELOW ME AND SO I REQUESTED DIRECT TO LUBBOCK; TX. I WAS CONFIDENT THIS WOULD KEEP ME ABOVE THE CLOUDS AND AWAY FROM A SNOW STORM WHICH WAS TO BE NEAR AND N OF AMARILLO. THE CTLR ASKED WHY NOT DIRECTS SNL? I TOLD HIM I WANTED TO STAY S OF THE WX. HE STATED HE SAW NO MOISTURE. SO I THEN REQUESTED DIRECT SHAWNEE. NIGHTTIME FELL JUST BEFORE AMA. THE WERE REPORTING SNOW. AS I PROCEEDED ABOUT 40 MI E OF AMARILLO; I ENCOUNTERED A FOG-LIKE REFLECTION IN MY WINGTIPS; SO I CLBED TO 13000 FT. TEMP THERE WAS 2 DEGS F. EVERY 10 MINS I OBSERVED FOR ICE ON MY WINDSCREEN AND WING SURFACES. I SAW NO ICE THE ENTIRE FLT. I ESTIMATED I MUST BE BTWN LAYERS WITH LIGHT SNOW ABOVE ME WHICH WAS UNABLE TO FREEZE ON THE WINGS OR AIRFRAME. I CALLED CTR AND ASKED ABOUT MOISTURE AND WX APCHING OKLAHOMA CITY. HE SAID HE SAW NO MOISTURE WHERE I WAS AND THAT HIS WX WAS SOME 50 MINS OLD. I NOTICED MY EGT WAS BEGINNING TO INCREASE AND MY CYLINDER HEAD TEMP WAS DOWN 25-50 DEGS. I ENRICHED MY FUEL MIXTURE BUT THE TEMP CONTINUED TO INCREASE. THE EGT BEGAN TO FLASH INDICATING EXTREME LEANNESS. I THOUGHT I MIGHT HAVE CONTAMINATED FUEL AND SO I INFORMED ZFW I WAS EXPERIENCING SERIOUS TROUBLE. I REQUESTED IMMEDIATE DSCNT AND INFORMED THEM MY ENG WAS BEGINNING TO RUN ROUGH. I INFORMED CTR IT APPEARED I MAY BE ABOUT TO GO DEAD STICK. CTR SUGGESTED HINTON; OKLAHOMA; 208; AS THE NEAREST ARPT FOR ME. I USED THE GPS TO FLY DIRECT. BASED ON THE ENG ALTERNATOR TONES AND THE LIGHT DIMMING; I SUGGEST MY ENG MY HAVE BEEN NEAR A FULL STALL AS MANY AS 3 TIMES DURING MY DSCNT FROM 13K TO 4K. I WAS CLRED FOR 6K AND THEN 4K. ONCE OVER THE ARPT; I REMEMBER BEING ABOVE 3500 FT BUT NOW WITH FULL PWR APPEARING TO HAVE BEEN RESTORED. I REMAINED AFRAID TO PULL THE PWR BACK FOR FEAR OF LOSING ENERGY THE ENTIRE DSCNT. AT SOME POINT GREATER THAN 4K I REMEMBER CTR SUGGESTING THAT I TURN THE LIGHTS ON AT THE ARPT AND ASKING OTHER PLTS TO ASSIST WITH LIGHT ACTIVATION. I WAS TOO BUSY CTLING THE ACFT TO RESPOND TO CTR OTHER THAN TO SAY THAT I REMEMBER THIS ARPT LEAVES THEIR LIGHTS ON ALL NIGHT. I MADE 3 APCHS TO THE ARPT BECAUSE I WAS UNABLE TO REDUCE MY AIRSPD SUFFICIENT TO LOWER GEAR OR FLAPS OR ACHIEVE LNDG SPDS. AFTER THE SECOND GAR; I WAS CONFIDENT THE ENG WAS OPERATING NORMALLY AS THE EGT AND CHT HAD RETURNED TO MORE NORMAL OPS. THE CEILING APPEARED TO BE ABOUT 2700 FT MSL WHEN I SAW GND. I LANDED NORMALLY WITHOUT INCIDENT OR ACCIDENT. NO ICE ON WINDSCREEN. SNOW WAS SEEN IN THE LNDG LIGHTS. RWY APPEARED BLACK. THIS WAS A REPEAT OF A SIMILAR EVENT 6 YRS EARLIER WHICH I MISTAKENLY CONTRIBUTED TO POSSIBLE WATER IN THE GAS LINE. THE FOLLOWING DAY; I CALLED MY MECH TO ASK ABOUT THE CAUSES. HE INFORMED ME OF THE POSSIBILITIES OF MOISTURE ENTERING THE ENG PAST THE AIR INTAKE FILTER AND THE BOOST AIR INTAKE DOOR NEAR THE LNDG LIGHT. THIS MOISTURE ENCOUNTERED IN 2 DEGS TEMPS MIGHT CREATE THE PROBS I ENCOUNTERED. HE SUGGESTED AVOIDING MOISTURE OF THE NATURE I ENCOUNTERED FOR THE MAKE AND MODEL I OWN WHEN UNDER 32 DEGS. I DID NOT REALIZE THAT MOISTURE COULD FREEZE INSIDE THE ENG INTAKE AREA AND WILL AVOID FLYING INTO MOISTURE OF ANY KIND BELOW FREEZING. I WAS EXPECTING TO BE ABOVE THE CLOUD LAYER BASED ON MY WX BRIEFINGS AND SHOULD HAVE TURNED AROUND WHEN I SAW SNOW IN MY LNDG LIGHTS NEAR AMA. I WAS UNAWARE THAT SNOW STORM TOPS COULD EXCEED 12000 FT AGL.