RV-6 pilot reported a loss of engine power during VFR initial climb from a non-tower airport. The pilot turned back to the airport as the engine was running erratically; made a radio call on UNICOM; and landed safely on the departure runway. Maintenance investigation determined the engine power loss was a result of a leaking fuel line.

Narrative

Planned to make a VFR flight from ZZZ during daytime in good weather. On the morning of the flight; checked NOTAMs; TFRs; weather; etc.; later at the airfield did a preflight; started the engine; did a runup; all were nominal. Just before takeoff on Runway XX; heard on UNICOM an aircraft report entering an extended 45 degree VFR entry to XX. Made a radio call stating my intention to depart straight out to the north; and commenced my takeoff roll. Shortly after takeoff; at approximately 400 ft. AGL over departure end of the runway; the engine abruptly lost power and began running erratically.Made a radio call on UNICOM that I was had a problem and was turning back; and got very busy maintaining control of the airplane; slowing to best glide airspeed; turning back towards the field; executing Emergency Procedures (EPs) such as richening the mixture; adding full carb heat; switching fuel tanks; confirming that the auxiliary boost pump was on; and maintaining control of the airplane. Mentally debated whether to try to land downwind on a reciprocal heading on Runway XY; but since the engine was developing at least partial power and since by then I was already halfway down the runway; decided that the safest thing to do was to fly a low; tight left downwind for [Rwy] XX; and land immediately. I turned a tight left base for XX and made a radio call saying so; and heard the other aircraft (which hadn't called entering downwind) also calling a turn for left base. I saw them turning base about a mile and a half farther out from me; so I got on UNICOM and clarified that I was also turning base; and was landing due to engine trouble. Heard no acknowledgment from them; landed without further incident; and made the first taxiway turnoff. Then saw the other aircraft on final and thanked them on UNICOM for their consideration; receiving two mike clicks as acknowledgment. (The other aircraft's proximity in flight to me was not within the 500 ft. minimum separation that would constitute a near miss under NMACS standards.).Found out later I had a leaking fuel line. Good thing I got it down promptly. Lessons learned from this would include that it would have been better for me to have [requested priority handling] on UNICOM and requested priority in the pattern over other traffic; and regardless; since I had declared a problem; for the other traffic to have called out their position in the pattern; to help with better situational awareness for both of us.