The pilot reported that, en route, he noticed a drop in the engine's rpm. He noted that the fuel shutoff valve was open, and the gas gauges showed half full in the right tank and quarter full in the left tank. He added that the carburetor heat was off, the mixture was full rich, and then the engine quit running. During the third attempt to restart the engine, it briefly started and then quit again. Subsequently, during an off-airport landing in a field, the airplane nosed over. The airplane sustained substantial damage to the empennage and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. He added that he had filled the gas "to the top" (22.5 gallons) before departure and had flown for 3.7 hours. The 1969 Cessna 150 owner's manual states that the airplane's maximum range was 4.1 hours with no reserve at 75% power at 7,000 ft. The pilot further reported in a telephone conversation with the National Transportation Safety Board investigator-in-charge that he did not consult the emergency checklist because it was placed in the glovebox. Federal Aviation Administration (FAA) inspectors drained the fuel tanks about 5 days after the accident and reported that there was no evidence of fuel leakage around the fuel caps or on the ground. They drained about 8 to 12 ounces from the left wing tank and about 3/4 of a gallon from the right wing tank. The FAA inspectors added that the engine showed proper continuity, and the magnetos were operational. The engine was not run due to a fractured intake manifold just above the carburetor base flange. The fracture damage to the intake manifold was consistent with impact damage.