On September 26, 2012, approximately 1500 mountain daylight time, a Beech E33, N7133N, registered to All Seasons Window Cleaning LLC, of Broomfield, Colorado, sustained substantial damage during a forced landing after a loss of engine power shortly after takeoff from the Greeley-Weld County Airport, Greeley, Colorado. The commercial pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The local flight was being conducted under the provisions of 14 Code of Federal Regulations Part 91. On the morning of the flight, the pilot/owner asked a local A&P mechanic to take a look at airplane's fuel system. The mechanic opened the engine cowling and found nothing out-of-the-normal visually. He then disconnected the fuel line between the fuel pump and the throttle body, unmetered fuel, and installed a remote pressure gauge to check to see where the fuel pressure was set. After removing the right side inspection cover of the lower cowling to access the throttle body, and while the pilot operated the engine from the cockpit controls, he found that the idle mixture was slightly rich and the fuel pressure at idle was within specifications. He readjusted the idle mixture setting at the throttle body and then checked the fuel pressure which was within specifications. The engine was shut down twice, restarted and the fuel pressure at idle and idle mixture settings were checked again. The pilot said that the engine ran so much better after making the idle mixture adjustments. After making final checks for fuel pressure and the locknuts on the idle mixture arm and an overall visual check of the engine compartment, the cowls were reinstalled. After a normal run-up, the pilot took off from runway 9 for a local test flight. The airplane was seen climbing out normally and started to circle the airport. The airplane then descended toward the approach end of runway 27 as the pilot declared an emergency due to engine problems. The airplane landed in plowed field gear up about 1/2 mile from the approach end of runway 27. After the accident, the airplane was moved to a local facility for further examination. FAA inspectors examined the wreckage and could not determine the cause of the loss of engine power. An engine run was attempted on January 3, 2013, at Beegles Aircraft in Greeley, CO. The engine was run on the airframe with a different, unbent slave propeller and taken through a series of tests up to full power. No anomalies were noted with the exception of slight fuel flow indicator fluctuations. There was nothing found that may have contributed attributed to the reported loss of power by the pilot.