On July 2, 2001, at 1734 hours Pacific daylight time, a Cessna 150, N6475T, made a forced landing in a grape vineyard near Delano, California, after a total loss of engine power. An individual was operating the airplane under the provisions of 14 CFR Part 91. The commercial pilot and one passenger were not injured. The airplane sustained substantial damage. The local area flight was operated as a public-use aircraft for the purpose of photographing crops for the U.S. Department of Agriculture. The flight departed from Delano at 1400. Visual meteorological conditions prevailed, and no flight plan had been filed. According to the pilot, his passenger fueled the airplane prior to departure. They purposely waited until after they ate lunch and immediately before departure to refuel the airplane in order to obtain the maximum quantity of cool fuel. The passenger had flown about 40 photo missions and was aware of the importance of fueling the airplane to maximum capacity. He assured the pilot that the fuel tanks were completely full. The accident occurred 3 hours 34 minutes after departure. The pilot stated that during recovery, 1/2-gallon of fuel was found in the airplane. The pilot also told the Safety Board investigator that he routinely flew the airplane 4 hours to 4 hours 5 minutes and it would require 20- to 20.5-gallons fuel to fill the tanks. The airplane holds 26 gallons fuel, of which 22.5 are usable fuel. The engine was test run at the facilities of Valley Aircraft, Tulare, California, under supervision of an Federal Aviation Administration (FAA) inspector from the Fresno Flight Standards District Office (FSDO). The engine started promptly and ran smoothly. The magneto check and carburetor heat check were normal. The engine did not stop when the mixture control was moved to the idle cutoff position. The inspection revealed that the mixture control arm was loose on its shaft. The carburetor was removed from the engine, without opening or disassembling it, and shipped to the facilities of Precision Airmotive Corporation. The carburetor was examined there, in the presence of an FAA FSDO inspector, on August 23, 2001. The examination revealed that the mixture control valve stem shaft was bent and was not inserted into the sleeve in the bowl. According to the representative from Precision Airmotive, the carburetor operates in the "full rich" condition at all times when the stem is not inserted in the sleeve, and any subsequent movement of the mixture control would have no effect. The examination also revealed that the mixture control lever was loose on the shaft and the stop peg traveled beyond the idle cutoff stop pad. On the flow bench, the full-rich fuel flow was within serviceable limits. The pilot/operator reported that no recent maintenance had been performed on the carburetor.