According to the pilot, during the preflight inspection, he checked the full fuel tanks for water at all three access points, with none found. The engine ran smoothly during taxi, run up and takeoff, but "stumbled" upon level off. The pilot switched tanks, "checked" the carburetor heat, and found that the engine only smoothed out when he added full power. The pilot decided to return to the airport, and during the base leg, the engine lost all power, resulting in a forced landing in a cornfield. A postflight examination of the airplane by a Federal Aviation Administration inspector revealed no obvious mechanical anomalies with the physical components of the engine. However, when he examined the carburetor fuel inlet filter, the inspector found it about 75 percent blocked with particulate matter consisting of large quantities of rust, insect remains, and sand. A restriction of this amount would significantly effect the engines ability to operate at lower power settings and could result in a total loss of engine power.