At approximately 1745 central time, a Cessna 150 lost partial engine power and crashed during the ensuing forced landing near Fort Worth. The pilot was not injured. The pilot said he was climbing through 50 feet agl when he noticed the engine starting to lose power. He continued to climb and flew a traffic pattern, turning to downwind at about 200 feet agl. He elected to land in a field ahead of him about a half-mile east of the runway. An FAA inspector reported the mixture control rod had separated from the carburetor.