A CORPORATE PLT EXPERIENCES ENG DIFFICULTIES SHORTLY AFTER TKOF AND RETURNS AND MAKES AN EMER LNDG.

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AFTER DEPARTING RLD IN MVFR CONDITIONS (HAZE; VISIBILITY 3 MI; 900 SCATTERED 1500 SCATTERED TO BROKEN) ON IFR FLT PLAN; ENG(S) BEGAN MISSING; RUNNING ROUGH AND SURGING. IMMEDIATELY ADVISED CHINOOK DEP CTLR (ATC) THAT I WAS MAKING A 180 DEG TURN TO RETURN TO ARPT BECAUSE I HAD ENG PROBS. ATC INITIATED CALLS TO ALERT EMER VEHICLES TO STAND BY AT RLD ARPT. ENG(S) CONTINUED TO RUN ROUGH THROUGHOUT REMAINDER OF FLT INCLUDING LNDG. BUT; ROUGHNESS AND SURGING DECREASED IN SEVERITY AS PWR WAS REDUCED FOR DSCNT FROM APPROX 5000 FT MSL TO APPROX AT 2000 FT MSL AND SUBSEQUENT LNDG. FUEL WAS ADDED APPROX 45 MINS PRIOR TO FLT AND CHKED APPROX 20-30 MINS LATER WITH NO CONTAMINATION OBSERVED. HOWEVER; AFTER LNDG A MECH CHKED FUEL AND SAID HE SAW TINY DROPS OF WATER IN MAIN TANK SAMPLE; BUT NONE IN OTHERS (2 WING TANKS AND AUX TANK THAT FEEDS MAIN TANK). HE SAID THAT OTHER ACFT HAD RPTED PROBS WITH FUEL CONTAMINATION (WATER) ON SEVERAL OCCASIONS. AFTER SEVERAL RUN-UPS; FUEL CHKS; TAXIES; FUEL CHKS; ETC; A TEST FLT WAS MADE WHEN ALL APPEARED NORMAL. FLT WAS NORMAL; NO ENG PROBS. LATER; ANOTHER IFR FLT WAS ATTEMPTED IN VFR CONDITIONS AND ENG(S) STARTED SURGING SHORTLY AFTER LIFT-OFF. AN IMMEDIATE RETURN FOR AND LNDG WAS MADE. ACFT IS NOW IN SHOP BEING EXAMINED FOR FUEL; TURBO; INDUCTION; ETC; PROBS. ACFT RECENTLY HAD ANNUAL INSPECTION BY MANUFACTURER (AEROSTAR ACFT CORP); INCLUDING OVERHAUL OF ONE CYLINDER AND ENG WING DEICE UPGRADES TO 'KNOWN ICING CERTIFIED.' ALL APPLICABLE MANUFACTURER SAFETY BULLETINS; AND FAA AIRWORTHINESS DIRECTIVES WERE COMPLIED WITH AT THAT TIME (JAN/XX/96 TO FEB/XX/96). WATER IN FUEL CONTAMINATION WAS RULED OUT AS CAUSE ON THIS SECOND FLT. ON SUBSEQUENT FLT; WITH MECH ON BOARD; TWO VERY BRIEF ENG MISSES WERE NOTED AFTER TKOF AND AT PATTERN ALT AS PWR WAS BEING SLOWLY REDUCED. ENGS ARE LYCOMING 10-540'S WITH MACHEN UPGRADES TO 350 HP (42 IN MAP @ 2500 RPM) WITH APPROX 550 HRS SINCE MAJOR OVER- HAULS. CAUSE IS UNDER INVESTIGATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS ACFT WAS A MODEL 601T AEROSTAR WITH MODIFICATIONS THAT BROUGHT IT UP TO THE SUPERSTAR MODEL 700 PERFORMANCE ACCORDING TO THE RPTR. AFTER SEVERAL UNSUCCESSFUL TEST FLTS THE MECHS CONTACTED THE ENG MODIFICATION CORPORATION AND THEY RECOMMENDED THAT THE INJECTORS BE CLEANED OR CHANGED. THE FAULT WAS FOUND TO BE PRIMARILY IN THE L ENG'S INJECTORS. THERE WERE 2 DIFFERENT STYLES OF INJECTORS AND THE ONES ON THE L ENG WERE THE 1-PIECE TYPE. THIS TYPE CANNOT BE DISASSEMBLED FOR CLEANING AND INSPECTION WHEREAS THE 2-PIECE TYPE ON THE R ENG CAN BE EXAMINED. AFTER THE L ENG'S INJECTORS WERE CHANGED TO NEW 2-PIECE TYPE INJECTORS IT PERFORMED WELL.