Narrative:

During a single pilot IFR flight from bed, ma, beginning descent following radar vectors in IMC for the VOR approach into gai the engine of the single engine small aircraft which I was plting began to 'run rough' and loose power. Since I had already reduced power to 2300 RPM for a cruise descent in the clouds, full carburetor heat had already been applied. I tried increasing the RPM to 2400 RPM and (although fuel pressure appeared normal) turned on the electric fuel boost pump and set mixture to full rich. When engine performance did not improve, I switched from the left fuel tank to the right tank at which time engine performance began to improve. I concluded that one of 2 problems existed; either the left tank was almost empty or the left tank contained sufficient water to affect engine performance. (The left fuel gauge showed between 1/4 and 1/2 tank, the right fuel gauge showed just over 1/2 tank and the mixture had been leaned using the egt for the flight). Before departing from bed I had checked the fuel tanks and drained fuel from the 3 specified points and found an unusually large amount of water in the left tank requiring several drainings to clear. The tanks were down equally 1-2 inches (the log shows 1.2 hours flown the day before). I called to have a truck top off the tanks while I received a preflight WX briefing and filed my flight plan. Approximately 25 mins after refueling I rechked the tanks (both full) found a small amount of water in the left tank which I cleared. I waited an additional 20 mins, found no water and decided to go.

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Original NASA ASRS Text

Title: PVT PLT IN SMA ENROUTE TO GAI HAD ENGINE RUN ROUGH. DIVERTED LANDED BWI. WATER IN LEFT TANK.

Narrative: DURING A SINGLE PLT IFR FLT FROM BED, MA, BEGINNING DSNT FOLLOWING RADAR VECTORS IN IMC FOR THE VOR APCH INTO GAI THE ENG OF THE SINGLE ENG SMA WHICH I WAS PLTING BEGAN TO 'RUN ROUGH' AND LOOSE PWR. SINCE I HAD ALREADY REDUCED PWR TO 2300 RPM FOR A CRUISE DSNT IN THE CLOUDS, FULL CARBURETOR HEAT HAD ALREADY BEEN APPLIED. I TRIED INCREASING THE RPM TO 2400 RPM AND (ALTHOUGH FUEL PRESSURE APPEARED NORMAL) TURNED ON THE ELECTRIC FUEL BOOST PUMP AND SET MIXTURE TO FULL RICH. WHEN ENG PERFORMANCE DID NOT IMPROVE, I SWITCHED FROM THE L FUEL TANK TO THE R TANK AT WHICH TIME ENG PERFORMANCE BEGAN TO IMPROVE. I CONCLUDED THAT ONE OF 2 PROBS EXISTED; EITHER THE L TANK WAS ALMOST EMPTY OR THE L TANK CONTAINED SUFFICIENT WATER TO AFFECT ENG PERFORMANCE. (THE L FUEL GAUGE SHOWED BTWN 1/4 AND 1/2 TANK, THE R FUEL GAUGE SHOWED JUST OVER 1/2 TANK AND THE MIXTURE HAD BEEN LEANED USING THE EGT FOR THE FLT). BEFORE DEPARTING FROM BED I HAD CHKED THE FUEL TANKS AND DRAINED FUEL FROM THE 3 SPECIFIED POINTS AND FOUND AN UNUSUALLY LARGE AMOUNT OF WATER IN THE L TANK REQUIRING SEVERAL DRAININGS TO CLR. THE TANKS WERE DOWN EQUALLY 1-2 INCHES (THE LOG SHOWS 1.2 HRS FLOWN THE DAY BEFORE). I CALLED TO HAVE A TRUCK TOP OFF THE TANKS WHILE I RECEIVED A PREFLT WX BRIEFING AND FILED MY FLT PLAN. APPROX 25 MINS AFTER REFUELING I RECHKED THE TANKS (BOTH FULL) FOUND A SMALL AMOUNT OF WATER IN THE L TANK WHICH I CLRED. I WAITED AN ADDITIONAL 20 MINS, FOUND NO WATER AND DECIDED TO GO.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.