On May 27, 1999, about 0830 mountain daylight time, an Air Tractor AT-401, N6024M, registered to West One Bank and operated by Spray Craft, Incorporated, as a 14CFR137 aerial applicator, was substantially damaged during an off-airport landing following an abrupt loss of engine power, eight miles north of Jerome, Idaho. Visual meteorological conditions prevailed and no flight plan was filed. The commercial pilot was not injured. The flight originated from Jerome County Airport, Jerome, Idaho, approximately one hour prior to the accident. During a telephone interview and subsequent written statement, the pilot reported that he departed the airport and proceeded to the spray location, approximately 19 miles northeast of Jerome with approximately 75 gallons of fuel on board. Approximately 45 minutes into the flight, the pilot noticed the fuel quantity gauge indication was decreasing at a higher than normal rate, and he elected to return to the airport. Approximately eight miles from the airport, the aircraft began to lose power and the pilot elected to initiate a forced landing in an open field. During the landing roll, the aircraft struck a lava bed, causing substantial damage to the aircraft's landing gear, engine and firewall. A Federal Aviation Administration inspector from the Boise, Idaho, Flight Standards District Office inspected the aircraft after the accident and reported that there was no evidence of fuel in the aircraft's fuel tanks. He also stated there was no evidence of a preexisting fuel leak. According to the pilot, the aircraft "experienced an abnormally high fuel burn due to either a stuck float in the carburetor or [a] malfunctioning carburetor mixture control." The fuel consumption rate for this aircraft is approximately 32 to 35 gallons per hour under normal conditions. Post-accident inspection of the aircraft's carburetor (serial number 5627175) revealed that the mixture control back-suction bleed tube (refer to Diagram I) in the carburetor bowl assembly was plugged. A back-suction type mixture control leans out the mixture by placing a portion of the venturi suction on the fuel in the float chamber so that it opposes the suction in the venturi on the main discharge nozzles. According to the certificate holder of the carburetor (Precision Airmotive Corporation of Everett, Washington), a blockage of the mixture control back-suction bleed tube would result in a rich mixture situation. Based upon the pilot's statement, fuel consumption during this flight would have been 60-75 gallons per hour. Normal fuel consumption for this aircraft is 32-35 gallons per hour.