On July 8, 2008, about 1240 mountain daylight time, a Cessna 150F, N8647G, experienced a partial loss of engine power during initial climb following a touch-and-go landing at the Kalispell City Airport, Kalispell, Montana. The pilot attempted a forced landing in a nearby field. During approach over uneven terrain, the airplane impacted the ground hard and collided with a fence. The airplane was destroyed by impact forces and post impact fire. The certified flight instructor (CFI) and student pilot sustained serious injuries during the instructional flight. The airplane was owned by a private individual, and it was operated by the CFI under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the round robin flight that began from Kalispell, at an undetermined time. In the pilot's "Aircraft Accident Report," he stated that after taking off from the Kalispell City Airport he and his student flew to Glacier Park International Airport where "approximately 10 touch-and-goes were performed." Thereafter, they flew back to Kalispell and performed two more touch-and-goes. The accident occurred during initial climb following the third takeoff. The CFI reported that the "engine sputtered" just before reaching the departure end of runway 13. The CFI took the controls and manipulated the engine's throttle. Following this action, the airplane seemed to resume climbing. Upon reaching between 400 and 500 feet above the ground and turning onto the crosswind leg, the engine lost a significant amount of power. The airplane's pitch attitude decreased, and despite the CFI's efforts to maintain flight, the airplane descended into the ground. The airplane came to rest an estimated 1,000 feet beyond the runway's departure end. Several witnesses observed the accident. One of the witnesses was a mechanic who was located next to the airport. The witness reported that he saw the airplane take off and heard the engine sputter and quit when the airplane was about 30 feet above ground level. Thereafter, he heard the engine restart when the airplane was between 200 and 300 yards past the departure end of the runway and was climbing. The witness observed the airplane begin a right turn, and the engine lost power again. The airplane quickly descended in a slight nose down attitude. The witness additionally reported that he did not see the actual crash because buildings blocked his view. The witness observed smoke rising from the crash site area 2 to 3 minutes after the crash. A bystander rescued the crew from the airplane wreckage prior to it being destroyed by fire. Participants from the Federal Aviation Administration (FAA), Cessna Aircraft Company, and Teledyne Continental Motors examined the airplane. In part, the FAA reported that during the examination the carburetor's accelerator pump functioned, and evidence of fuel was observed. The residual fuel was field examined. It did not have a blue tint, consistent with 100LL aviation fuel, and it did not smell like avgas, according to the FAA's principal maintenance inspector. No evidence of any preimpact internal engine or airframe component failure was reported by the parties. At the direction of the National Transportation Safety Board's investigator, the FAA sent the airplane's Precision carburetor, Model MA-3SPA, s/n BE02451, for examination to the Safety Board's Office of Research and Engineering, Materials Laboratory, Washington, D.C. Accompanying the carburetor was its associated air inlet ducting, air filter, and a sample of the airplane's fuel. The Safety Board's laboratory examined the submitted evidence. Laboratory personnel reported finding granular material on the walls of the carburetor throat and interior of the inlet duct. The granular material appeared to be a mixture of very fine dust-like particles and slightly larger multi-colored rounded sand particles. The larger particles were approximately 0.020 to 0.040 inch in diameter, while the fine material was less than 0.001 inch in size. Upon disassembling the carburetor, the idle jet was found substantially blocked with the fine granular material, and the main jet was also partially obstructed with similar granular material. The fuel examination revealed a mixture of plastic shavings and clumps of mixed fine and coarser granular material. The material was similar to that found inside the carburetor jets. The airplane's owner reported to the Safety Board investigator that the airplane is not frequently used. A review of the airplane's maintenance record logbook indicates that the airplane received its most recent annual inspection on February 22, 2008, at a total time of 4,228.7 hours. About 10 months earlier, on April 24, 2007, the airplane's total time was 4,219.9 hours. It had been operated 8.8 hours during this period. The FAA provided the Safety Board investigator with a fuel receipt bearing the accident airplane's registration number, dated July 3, 2008. The receipt documented the acquisition of 18.54 gallons of aviation fuel at an airport in Montana.