Date: 201506
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: Marginal
Weather elements / visibility: Cloudy; 10
Light: Daylight
Aircraft: M-20 F Executive 21
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Initial Climb
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Human factors: Training / Qualification
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Weather
Primary problem: Human Factors

Narrative: I landed at [airport X] following an IFR flight from another airport. Engine performed normally. After arriving; we parked at FBO where we requested a top off (100LL) and one quart of oil. Rain had just begun to fall when we landed; and by the time we parked it was raining very hard. I observed the line crew fueling the plane at this time; holding an umbrella over the fuel tanks while doing so. However; the rain was falling so hard that it was difficult to keep the umbrella in place. We returned to the airport a few hours later and prepared for departure. After a preflight; I obtained an IFR clearance and taxied for departure to Runway 25. The runup was uneventful with the engine running smoothly and producing full power. I was cleared for takeoff; and acceleration was normal. Shortly after rotation (about 200 AGL) the engine began to run very rough but continued to produce just enough power to continue the climb. Not knowing how much runway was left; I elected to continue the climb straight ahead. I immediately notified tower that the engine was running rough and I needed an immediate return to the field. I turned to the right and was cleared to land on runway 10 (my best option). After making the turn I elected to maintain my altitude (below the cloud deck) instead of risking losing all engine power. At least I was over a large airport for safety. I requested from tower that I hold my altitude and attempt to troubleshoot the problem; and was advised to proceed eastbound and remain VFR; which I did. While at the midfield position; the engine cleared and I was able to continue my flight uneventfully. I've experienced water in my tanks before; and this was a clear case of the same problem. We had a fortunate outcome this time. My issue was with the FBO crew who fueled the plane. They elected to fuel my plane while in a heavy rainstorm; which dissipated not long afterwards. I was at [airport X] for a few hours; so there was no rush to fuel the plane immediately. I feel that their decision to fuel the plane immediately during a heavy rainstorm introduced water into my tanks which nearly resulted in an emergency landing; possibly off field. The tower crew did a great job and was able to keep me where I needed to be; and safely away from other traffic as well. In retrospect; I should have sumped the fuel during my preflight; but I thought the few hours I was parked was not enough for the water (if present) to separate from the fuel enough to be removed.  I had not considered the possibility of rainwater being introduced into the fuel during the fueling process; which is was I believe happened.




Synopsis: M20F pilot reported a rough running engine for a few moments after takeoff. Reporter stated the aircraft was refueled in a heavy rain.