On November 16, 2010, about 0940 eastern standard time, a Cessna 172, N8306B, was substantially damaged during a forced landing following an in-flight loss of engine power near Hobe Sound, Florida. The airplane was operated by a private individual. The certificated airline transport pilot was not injured. Day, visual meteorological conditions prevailed at the time, and no flight plan was filed for the personal flight from Jupiter, Florida to Jacksonville, Florida (CRG). The flight originated at Tailwinds Airport (FD15) at 00915. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. The pilot reported that about 20 minutes into the flight, the engine began to lose power. He informed air traffic control of the situation and requested a vector to the closest airport. He was able to keep the engine running by actuating the fuel primer knob, but the airplane would not maintain altitude. The pilot performed an emergency landing in an orange grove near Hobe Sound. The airplane touched down in soft terrain and nosed over. An inspector with the Federal Aviation Administration responded to the accident site and examined the wreckage. He reported that the wings sustained substantial damage. During recovery of the wreckage, he confirmed positive fuel flow to the engine and the fuel tank vents were clear. All fuel screens were clear and the air intake was not obstructed. The throttle, mixture, and carburetor heat controls operated normally. The spark plugs were removed and the electrodes exhibited normal wear and color. The engine was turned by hand and internal continuity was confirmed. Compression was observed an all cylinders. The carburetor was removed and the accelerator pump operated normally when actuated by hand. Fuel was present in the carburetor bowl. Disassembly of the carburetor revealed the needle valve and seat were undamaged. A thick, slimy substance, black and brown in color, was found in the carburetor bowl drain plug and the substance was obstructing the fuel channel in the carburetor. The origin of the contamination was not determined. A review of the engine logbook revealed that the last annual inspection was performed on August 16, 2010. No discrepancies with the carburetor were noted. At the time of the accident, the airplane had been operated about 11 hours since the last annual inspection. Weather, recorded at 0953 at West Palm Beach, Florida (PBI), located about 25 miles southeast of the accident site, included winds from 200 degrees at 10 knots, visibility 10 statute miles, few clouds at 3,500 feet, temperature 27 degrees C, dew point 17 degrees C, an altimeter setting of 29.97 inches Hg.