On June 29, 2013, about 1430 Alaska daylight time, a Cessna 175 airplane, N7698M, sustained substantial damage during a forced landing, following a partial loss of engine power near Skwentna, Alaska. The airline transport rated pilot and one passenger were not injured. The airplane was registered to, and operated by the pilot as a visual flight rules personal cross-country flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed. The airplane was originating from the Yentna Lodge Strip, destined to Merrill Field, Anchorage, Alaska. According to the pilot, he was departing from the grass-covered runway, and at approximately 100 feet above the ground, the airplane's engine began to lose power. Unable to maintain altitude, the pilot landed the airplane in a swamp, and during the landing the airplane nosed over, substantially damaging the wings and vertical stabilizer. The pilot stated that the engine had just been returned to service after a major overhaul, and that it had operated approximately 3 hours since being returned to service. Examination of the engine logbooks revealed that the engine was removed from the accident airplane on May 4, 2011 at a tachometer time of 209.2 hours. The engine was overhauled on March 1, 2012, and installed in the accident airplane on June 1, 2013 (same tachometer time). At the time of the examination, the tachometer read 212.5 hours. On October 31, 2013, the National Transportation Safety Board investigator-in-charge, along with an investigator from Continental Motors, examined the wreckage at the facilities of Alaska Claims Servies, Wasilla, Alaska. No preaccident anomalies with the airframe were noted. Examination of the engine revealed that manual rotation of the propeller resulted in a spark on each of the spark plugs, in firing order, with the exception of the No. 6 top spark plug. Testing of the No. 6 top spark plug, while attached to a different ignition lead, indicated that the spark plug functioned properly. Testing of the magneto's distributor cap revealed that the ignition tower for the No. 6 top spark plug functioned appropriately. Testing of the No. 6 top spark plug's ignition lead after it was removed and repositioned on the distributor cap revealed that the lead was inoperative. Examination of the lead revealed an area near the distributor cap that was permanently distorted and pinched. There was noticeable buckling and denting on the firewall aft of the magneto distributor cap, but it could not be determined whether the damaged lead was caused by a momentary impingement from the firewall. The carburetor was removed from the engine and was disassembled. Disassembly of the carburetor revealed no internal anomalies or debris. All of the orifices were clear and clean. The fuel inlet screen was removed and no debris was noted. The glass fuel strainer was approximately 1/3 full with an amber colored fluid that appeared dirty. The fluid was collected in a glass jar and water was detected in the sample using water finding paste (SAR-GEL). The fuel strainer screen was dirty but not blocked. It could not be determined if the water in the fuel strainer was present prior to the accident. According to the personnel that recovered the airplane from the accident site, there was at least 30 gallons of aviation fuel removed from the airplane. No other preaccident mechanical malfunctions or failures associated with the engine were noted that would have precluded normal operation.