The pilot reported that engine power decreased smoothly in cruise flight, at a power setting of 2100 RPM. The carburetor heat was applied for about one minute, then removed. He slowed the airplane down and executed an intentional stall into the trees. The weather at the time of the accident was favorable for the formation of carburetor ice, and the pilot said that he did not receive a weather briefing. Later, the pilot stated that he believed that carburetor ice had formed because during the engine teardown there was a lot of moisture in the carburetor.