Main metering jet unscrewed
The cause of the engine failure has been announced: "Examination of the airplane's engine revealed that the carburetor's main metering jet was unscrewed from its seat and rotated 90 degrees. The unseated jet would have allowed an increased fuel flow through the main metering orifice, producing an extremely rich fuel-to-air ratio, which would have resulted in the loss of engine power."
For the complete report search for NTSB Identification: WPR15FA121
RYAN AERONAUTICAL ST3KR, registration: N53178