The airplane nosed over during a forced landing following the partial loss of engine power in the traffic pattern. The pilot reported that while on downwind he reduced engine power to initiate a descent. While on base leg the airplane descended below glide path. The engine did not respond to multiple throttle inputs and continued to operate at 1,500 rpm regardless of throttle position. The airplane was too low to make the airport and a forced landing was made to a cornfield. The airplane nosed over during landing. Twelve gallons of fuel were drained from the airplane after the accident. The fuel selector was positioned on the left tank, the mixture control was mid-travel, and the carburetor heat was "off." The throttle, mixture, and carburetor heat controls functioned properly. A post accident inspection did not reveal any anomalies consistent with a loss of engine power. The temperature and dew point in the vicinity of the accident site were 20 degrees and 17 degrees Celsius, respectively. Data indicated the possibility of moderate carburetor icing at cruise power and serious icing at descent power under those conditions.