On March 12, 1999, approximately 1650 mountain standard time, a Piper PA-28-140, N9578W, registered to DB Rents of St. Anthony, Idaho, departed St. Anthony for Rexburg, Idaho (9 nautical miles southwest of St. Anthony), on a 14 CFR 91 personal flight. The aircraft did not arrive in Rexburg as expected, and a search for the airplane was initiated by friends of the pilot. The substantially damaged wreckage of N9578W was spotted from the air approximately 2 hours after N9578W's departure from St. Anthony, located off the north end of the Rexburg airport runway. Upon arrival of aid personnel at the accident scene, the private pilot-in-command, the airplane's sole occupant, was found seriously injured in the aircraft wreckage. Visual meteorological conditions prevailed, and no flight plan had been filed for the flight. An emergency locator transmitter (ELT) radio signal, which was noted to be strongest within approximately 1 mile of the Rexburg airport, was reported to have been heard by searchers during their search for the missing aircraft. The pilot reported to the NTSB that the accident occurred approximately 1700. The pilot's NTSB accident report indicated that the pilot remembered "nothing of the flight, the engine problems prompting him to prepare for an emergency landing, [or] the crash...", and that the pilot believed the accident was due to "engine failure due to unknown causes." Rexburg police officers who responded to the accident scene reported they observed apparent impact marks from the aircraft on a gravel bar in the middle of the Teton River, as well as on the south bank of the river (NOTE: the Teton River runs in a generally east-west direction adjacent to the north end of Rexburg runway 17/35.) The aircraft wreckage was located approximately 10 feet south of the south bank of the river and approximately 50 feet north of the north end of the runway, and had come to rest on a generally southwesterly heading. The police report indicated that at the accident scene, fuel was found in both of the aircraft's fuel tanks, and that the aircraft's fuel selector valve was found in the OFF position. A mechanic of Rexburg Air Service, Rexburg, Idaho, reported to the NTSB that he removed the carburetor float bowl drain plug following the accident, and found 1/2 teaspoon of fuel in the float bowl. Photos of the accident aircraft's instrument panel taken by the Rexburg Police Department at the accident site showed the aircraft's carburetor heat selector pushed in (i.e., in the "off" or "cold" position.) The FAA reported that the aircraft did not receive any air traffic services, and personnel at the Rexburg airport reported to the NTSB that they did not recall having any contact with N9578W on the airport UNICOM frequency (122.8 MHz) prior to the accident. At the request of the NTSB investigator-in-charge (IIC), an inspection authorization (IA) mechanic from Aerohawk Aviation, Idaho Falls, Idaho, traveled to Rexburg and performed a follow-up examination of the accident aircraft's Lycoming O-320-E2A engine. This examination was performed during the week of July 26-30, 1999. The Aerohawk Aviation IA mechanic's examination report stated: "I found no mechanical reason for the engine to stop running, as it appeared to be in pretty good shape." Photos taken during the follow-up examination showed one blade of the accident aircraft's 2-blade/fixed pitch propeller bent back about 30 degrees at mid-span, and the other blade bent back very slightly (about 5 degrees) at about 1/3 blade length inboard from the blade tip. Damage to the propeller appeared otherwise minimal in the photographs. The aircraft's Precision Airmotive MA-4SPA carburetor was sent to the facilities of the carburetor manufacturer, Precision Airmotive Corporation of Everett, Washington, for a flow check and disassembly examination. This examination took place on October 1, 1999. Precision Airmotive's report of this examination stated that during the flow check, the "unit did not flow at first until tapped on [its] side", but then flowed within design flow specifications. The disassembly examination revealed that the brass float (part number 30-766) had no retractor clip installed, and that an "old style" retractor clip (part number 29-193, specified for use with composite floats) was found laying in the bottom of the float bowl chamber. According to a Precision Airmotive technical representative, "The retractor clip is a device used to ensure that the float valve opens when the float drops. It attaches the valve to the float." The part number 29-193 retractor clip is used with an older composite float (part number 30-759), which was replaced with the "new" style brass float (part number 30-766) under Precision Airmotive Service Bulletin MSA-1. The installation instructions for the float kit used to install the part number 30-766 brass float (the float kit's part number is 666-915) specify use of a different retractor clip (part number 29-194, which is included in the 666-915 float kit along with the brass float) with the brass float, and instruct the installer to discard the old 29-193 clip before installing the new float and clip. The Precision Airmotive technical representative reported: "It is possible that the valve could stick shut if the clip were not in place." Rexburg-Madison County Airport, the accident aircraft's destination, is equipped with an Automated Surface Observation System (ASOS). The 1653 Rexburg automated weather observation reported the following conditions: wind variable at 4 knots, visibility 10 statute miles, clear skies, temperature 2 degrees C (36 degrees F), dewpoint minus 3 degrees C (27 degrees F), and altimeter setting 30.14 inches Hg. According to the engine log, at the time of the aircraft's last annual inspection (May 19, 1998) the engine had 2,773.49 hours total time and 278.28 hours since major overhaul.