AEROSTAR ON APCH TO SLC TWR REQUESTED GAR. ON ADDITION OF FULL PWR; ENG BEGAN TO RUN ROUGH; MISS AND SURGE.

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WHILE MAKING AN INST APCH ON THE ILS RWY 34R TO SLC IN IMC (MODERATE TO HVY SNOW; INDEFINITE CEILING 300 FT; VISIBILITY ON GND APPROX 1 MI) TWR TOLD ME TO EXECUTE A GAR JUST PRIOR TO REACHING DECISION HT (200 FT AGL). ON BRINGING PWR TO FULL THROTTLE; ONE OR BOTH ENGS BEGAN TO RUN ROUGH; MISS; AND SURGE. SUSPECTING ENG INLET ICING; I OPENED THE ENG INLET ALTERNATE AIR DOORS WITH NO NOTICEABLE EFFECT. A QUICK ENG SCAN WAS MADE; AND NO ABNORMAL INDICATIONS WERE NOTICED. SINCE TWR WAS GIVING CLB AND TURN INSTRUCTIONS TO RE-ENTER FLOW OF TFC INTO SLC; I DECLARED AN EMER. ENG(S) CONTINUED TO RUN ROUGH AND SURGE UNTIL PWR REDUCED FOR ILS RWY 34R APCH AGAIN. ON LNDG WHEN TURNING OFF RWY; R ENG QUIT. LCL SHOP FOUND NO ENG PROB. FLT WAS FIRST LEG OF IFR FLT TO ABQ. ACR USED FOR SLC TO ABQ LEG AND MY PLANE WAS LEFT AT SLC; EVEN THOUGH SHOP FOUND NO PROB AND A SHORT FLT IN PATTERN AT SLC WAS NORMAL (ALTHOUGH R ENG QUIT AFTER LNDG; BUT WAS RESTARTED AND RAN OK). DURING SHOP'S INSPECTION OF ENGS; SEVERAL PHONE CALLS WERE MADE TO MANUFACTURER AND 2 MECHS SPECIALIZING IN AEROSTARS. GENERAL CONCERN WAS THAT ENG ROUGHNESS; MISS AND SURGE ON GAR WAS DUE TO SNOW/ICE BUILDUP ON ENG INLET SCREENS AND/OR EXCESS RICHNESS OF FUEL-AIR MIXTURE DUE TO HIGH ALT OF SLC ARPT (APPROX 4650 FT MSL). ENG QUITTING AFTER LNDG WAS ATTRIBUTED TO EXCESS RICHNESS OF FUEL-AIR MIXTURE. UPON RETURN TO SLC FROM ABQ; AIRPLANE WAS FLOWN BACK TO HOME BASE POINT OF ORIGIN (RLD) IN VFR CONDITIONS. ENGS RAN NORMALLY (DATE OF THIS FLT FEB/XX/96). A SUBSEQUENT FLT ON MAR/YY/96 WAS BEING MADE FROM RLD TO RNT; WA; IN MVFR CONDITIONS ON AN IFR FLT PLAN; WHEN SIMILAR ENG PROBS WERE EXPERIENCED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE HAS LEARNED THAT THESE HIGH BOOST ENGS ARE HIGHLY TUNED AND NEED FREQUENT ATTN. THE ACFT IS NEW TO RPTR AND PART OF THE PURCHASE WAS THAT THE MANUFACTURER WOULD GIVE IT AN ANNUAL AND ASSURE THAT ALL AIRWORTHINESS DIRECTIVES WERE COMPLIED WITH. HE WAS QUITE SURPRISED TO HAVE TROUBLE IN SLC. RPTR TALKED DIRECTLY TO A COMPANY REPRESENTATIVE WHO GAVE HIM SOME FURTHER INSIGHT. HE HAD ANOTHER SUCH INCIDENT FROM HIS HOME BASE ON TKOF UNDER SIMILAR CONDITIONS BUT THE ENG WAS SURGING AT HIGH PWR. ONCE PWR WAS REDUCED FOR APCH (HE DID A RETURN LAND PROC); THE ENG SMOOTHED OUT. HIS MECH IS CURRENTLY REPLACING ALL INJECTORS; THEY WERE QUITE FOULED. THE MANIFOLD PRESSURE BOOSTER HAS BLEED AIR LINES WHICH MAY HAVE WORN O-RINGS; THEY ARE BEING REPLACED. THE MIXTURE IS BEING ADJUSTED QUITE CRITICALLY AS IT WAS TOO RICH. THE MECH IS GOING THROUGH EVERYTHING HE CAN TO ASSURE NO FURTHER PROBS. WHEN THE MECH SPOKE WITH MANUFACTURER MAINT PEOPLE HE GOT LITTLE IF ANY NEW INSIGHTS TO THE PROB. RPTR FEELS THIS IS JUST THE NATURE OF THE ENG DESIGN; IT IS SO HIGHLY TUNED AND NEEDS TO BE MONITORED CAREFULLY.