On October 28, 2006, about 1600 Pacific daylight time, a Fish Vans RV-6, N1198N, experienced a partial loss of engine power and collided with the ground during an attempted return to runway maneuver at the Turlock Municipal Airport, Turlock, California. The pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The private pilot and a passenger sustained serious injuries; the airplane sustained substantial damage. The local personal flight was originating at the time of the accident. Visual meteorological conditions prevailed, and a flight plan had not been filed. In a written statement, the pilot reported that following the departure roll on runway 30, he configured the airplane for the initial climb. After reaching about 500 feet above ground level (agl), the pilot noted a loss of engine revolutions per minute (rpm) and a decrease of airspeed. He maneuvered the airplane in a 180-degree turn in an attempt to return to the runway. The airplane stalled and the pilot attempted to recover. The airplane entered an accelerated stall again and descended rapidly. Upon impact, the airplane touched down hard, subsequently nosing over and coming to rest inverted. Following the accident, the pilot performed an examination of the engine in an effort to determine the cause of the partial loss of engine power. He noted that both spark plugs on the number 4 cylinder appeared to be oil fouled and the cylinder appeared as though it was not able to produce power. A further examination revealed that two of the piston rings were broken on the number four piston, allowing oil to move past the piston into the combustion chamber. He found the intake tubing for that cylinder loose. The pilot stated that the accident was "pilot error" as it could have been prevented if he had more engine-out practice.