On May 1, 1999, about 0920 central daylight time, a Cessna 172H, N5105G, registered to a private individual, nosed over after a forced landing in a field near Sylvan Springs, Alabama. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 instructional flight. The airplane was substantially damaged and the student pilot, the sole occupant, was not injured. The flight originated at 0905, from the Bessemer Airport, Bessemer, Alabama. The pilot stated that before takeoff, he preflighted the airplane with no discrepancies noted. The flight departed with full fuel tanks and after takeoff, he flew to the area of his mother-in-law's house, circled the area, flew to another area, and returned to the area of his in-law's house. He applied carburetor heat, reduced the throttle, for one last look, then pushed the throttle and carburetor heat controls; there was no response from the engine. He pulled the throttle control then pushed it again but there was no response from the engine. He later stated that he thought the engine was at idle rpm. He then verified that the magneto switch was in the "both" position, but did not move it, and verified that the mixture control was in the ""full rich" position, which had not been changed since takeoff. He then pumped the throttle control and the primer control, and the engine responded momentarily with each application, but not enough to maintain altitude. He began flying toward a field he noted and just before touchdown, he pulled the mixture control to the idle cut-off position, and placed the magneto switch to the "off" position. After touchdown, the airplane traveled through a barb wire fence then nosed over. He noted after exiting the airplane that fuel was leaking from the fuel tank vent line. The airplane was examination at the accident site by an FAA inspector who noted fuel leakage from both fuel tanks which contained automotive fuel. The airplane was recovered and a serviceable propeller was installed for the attempted engine run. Additionally, following the accident, the fuel tanks were drained but the drained fuel was added for the engine run. The engine was started, operated at idle for 5 minutes, then operated to 1,700 rpm where magneto check was performed. The left and right magneto drop was 60 and 50 rpm, respectively. Also, carburetor heat was applied and the engine rpm decreased 125 rpm. The throttle was increased full forward for about 15 seconds and the engine rpm increased to 2,300 rpm. The engine was then secured. Review of the carburetor icing probability chart revealed the conditions were favorable for moderate icing at cruise power or serious icing at descent power.