On May 10, 2007, about 1420 Pacific daylight time, a Cessna 152, N68166, experienced a partial loss of engine power during cruise flight, about 3 miles east of Monroe, Washington. The pilot made a hard forced landing in an open, grass-covered field. During rollout, the airplane's nose gear assembly and propeller sheared off. The airplane was substantially damaged, with a deformed firewall. It came to rest in a nose down attitude. The private pilot was not injured. The airplane was operated by Northway Aviation, Everett, Washington. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. The personal flight was performed under the provisions of 14 Code of Federal Regulations Part 91, and it originated from Everett about 1410. The pilot reported to the National Transportation Safety Board investigator that while cruising about 95 knots, at an engine speed of 2,300 rpm, the engine lost power. The engine began "running very rough," and the engine's speed decreased to 1,200 rpm. The pilot reported that he applied full carburetor heat and checked the magnetos, but he did not perceive any change in the engine's power. So, he prepared for a forced landing. The airplane touched down two times before coming to a stop in a nose down attitude. Under the direction of the Safety Board investigator, personnel from AvTech Services, LLC, Kent, Washington, partially disassembled the engine under the supervision of Federal Aviation Administration (FAA) personnel. In summary, AvTech personnel reported that internal engine continuity was confirmed with all four cylinders. Both magnetos produced spark during rotation of the crankshaft. The top spark plugs were removed and appeared in good condition with normal combustion coloration. The fuel gascolator was drained and about 1/2 cup of "blue" aviation-like fuel was observed. The oil filter was cut open and no metal was noted inside. Compression was unremarkable in three of the cylinders. The fourth cylinder exhibited leakage from its exhaust valve. No evidence of any mechanical malfunction was found. The FAA coordinator stated that he was unable to find evidence for the pilot's reported loss of engine power. Review of a carburetor icing probability chart disclosed that the temperature and dew point were within an area of the chart annotated "Moderate Icing -- Cruise Power or Serious Icing Descent Power."