On April 2, 2002, about 1645 Pacific standard time, a Cessna 152, N48908, made an off airport forced landing following a loss of engine power immediately after departing Boulder City Municipal Airport, Boulder City, Nevada. Air Excel, Inc., was operating the rental airplane as a local instructional flight under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and student pilot were not injured; the airplane sustained substantial damage. Visual meteorological conditions prevailed, and a flight plan had not been filed. In a written statement, the CFI stated that after departure, upon reaching 300 to 400 feet above ground level (agl), the engine experienced a rapid loss of power. He maneuvered the airplane in a gliding configuration and, after determining that the airplane was unable to maintain altitude, turned back toward the airport. While turning, he attempted to troubleshoot the partial loss of power, by confirming the throttle and mixture control were in the full forward position, the fuel selector was situated to "on," the magneto selector was positioned on "both," and the primer knob was locked; he manipulated the carburetor heat to the on position. As the airplane approached the airport, he realized that he would be unable to clear power lines located directly on his flight path. He opted to perform an off-airport landing and configured the airplane with full wing flaps. While in the landing flare, the airplane collided with vegetation and continued across a dry riverbed. The airplane came to an abrupt stop after impacting a large bush. AIRCRAFT INFORMATION The Cessna 152, serial number 15281043, was manufactured in 1977, and had accrued a total time in service of 7,073 hours. The most recent annual inspection was completed on January 18, 2002, 154.8 hours prior to the accident; a 100-hour inspection was preformed on February 21, 2002, 61.4 hours prior to the mishap. The Lycoming O-235-L2C engine, serial number L-14245-15, had accrued 276.7 hours since the most recent major overhaul. Annual and 100-hour inspections were accomplished on the dates noted for the airframe. A review of the operator's aircraft status sheet and maintenance department records disclosed that on two separate occasions (February 21, 2002 and March 14, 2002) the engine failed to quit after the pilot attempted to shutdown the engine by manipulating the mixture control to idle cutoff. For both discrepancies, a mechanic stated that he took corrective action by adjusting the idle speed and corresponding idle mixture as specified in the Cessna 152 service manual. According to the operator, the airplane had completed several flights earlier in the day without mishap. There were no unresolved discrepancies at the time the pilot rented the airplane. He reported that, prior to departure, the airplane had an adequate amount of fuel for the intended training flight. In a telephone conversation with a National Transportation Safety Board investigator, the CFI reported that the airplane had a history of ongoing problems with the idle mixture. He further stated that in an effort to address those problems, the operator had the carburetor overhauled about a week prior the accident. In addition, maintenance personnel had adjusted the newly overhauled carburetor several days before the accident. TESTS AND RESEARCH Engine Examination. Following recovery, a Safety Board investigator examined the wreckage at Air Transport, Phoenix, Arizona. During the engine examination, investigators removed the top and bottom spark plugs, referencing their operational characteristics according to the Champion Aviation Check-A-Plug AV-27 Chart. Both spark plugs from the number one cylinder, as well as the top number two plug, appeared dark and sooty, consistent with carbon fouled conditions. The bottom number two plug was oily, which the chart states corresponds with oil fouled conditions; the top number three and four plugs were gray in color, indicating normal conditions; the bottom number three and four plugs were covered in lead deposits, consistent with lead fouled conditions. Investigators measured the carburetor heat control to be out approximately 1 inch, with its corresponding butterfly valve partially open. The throttle control linkage was bent, but upon manipulating the throttle control knob, it moved from the full open stop to the closed stop. The mixture control linkage moved freely from the full rich to the cutoff stops. The air filter was removed, and noted to be free of debris and particulates. The gascolator was full of a blue fluid, consistent in odor and appearance to that of aviation fuel; the gascolator screen was clean. After an initial failed attempt, investigators added ether in an effort to start the engine. The engine started, and continued to produce power as a technician manipulated the mixture and throttle controls to the full forward position. The engine accelerated up to 1,600 revolutions per minute (rpm) and, despite black smoke emitting from the exhaust stack, investigators noted it running smoothly. The technician then retarded the mixture control aft about 1 inch in attempts to lean out the fuel/air mixture. The engine accelerated to 2,100 rpm and the exhaust cleared, appearing less soiled. With the engine still at high rpm, the technician preformed a magneto test, by selecting one magneto to provide the sole electrical energy required to produce spark in each cylinder; the engine rpm dropped 100 rpm on each magneto. Carburetor Examination. The carburetor, an MA-3A (serial number CR-2-7501), was removed and shipped to Precision Airmotive Corporation, the Federal Aviation Administration (FAA) type certificate holder, at Marysville, Washington. An examination was preformed at their facilities under the auspice of a Safety Board investigator. An external inspection disclosed that the fuel inlet screen was absent from the carburetor, and the outer casing was broken near the inlet area. After a new inlet fitting was installed, the carburetor was affixed to the bench tester and flow pressure was applied. At the idle cutoff position, fuel flow was measured at 1.5 pounds per hour (lb/hr), as compared to the master flow of 5.5lb/hr. Investigators loosened the mixture adjustment screw by turning it 2 1/2 times, which increased the fuel flow up to 5.5lb/hr. The remainder of the flows at higher settings all tested within specifications. Disassembly of the unit revealed no mechanical malfunctions or anomalies.