On August 3, 2011, at 1000 mountain daylight time, an Ayres Corporation S2R-600, N4021S, lost partial engine power shortly after takeoff and the pilot force-landed the airplane in a field approximately 4 miles southeast of Hazelton Municipal Airport, Hazelton, Idaho. Crop Jet Aviation was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 137. The commercial pilot was not injured; the airplane sustained substantial damage. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that shortly after takeoff, the engine lost partial power. During the loss of power, the manifold pressure remained steady. He released the load of fertilizer and maneuvered over power lines to land on a road. The airplane was unable to sustain flight and the pilot landed the airplane in a field. The airplane impacted concrete obstacles, separating the landing gear from the fuselage, before coming to rest. Following the accident, the engine and propeller were examined by the Federal Aviation Administration inspector. The carburetor had been replaced 7 hours prior to the accident. About 160 hours prior to the carburetor replacement, half of the spark plugs had been replaced. Instead of the required Unison UREM40E, Unison UREM38E plugs were installed. The spark plugs appeared sooty. Continuity from the controls to the engine and propeller were intact. The magnetos were tested and operated normally. The engine was manually rotated with no problems identified. The carburetor was removed for further examination. Personnel at Gustin Aviation also examined the wreckage. The airframe fuel boost pump operated normally. Additionally, no evidence of fuel blockage throughout the system was present. The spark plugs were tested and found to be operational. The carburetor was examined at Precision Engines, Everett, Washington. Verification of the engine and carburetor combination was confirmed. Testing and disassembly of the carburetor revealed no operational anomalies. The carburetor icing probability chart from DOT/FAA/CT-82/44 Publication: Light Aircraft Piston Engine Carburetor Ice Detector/Warning Device Sensitivity/ Effectiveness, June 1982 showed a probability of serious icing at glide power at the temperature and dew point reported at the time of the accident.