Flight Details
A private pilot with 345 hours of flight experience conducted a detailed preflight inspection, determining that the airplane had 26 gallons of fuel on board for a cross-country flight. Departing from his home airfield, the pilot flew approximately 17 nautical miles to pick up a passenger at another airfield. He then continued to a third airfield located about 76 nautical miles away, with no problems noted during these segments. Approximately ten minutes after departing from the last airfield, the engine experienced a loss of RPM. The pilot ran the emergency checklist and searched for a suitable landing area as the airplane could not maintain altitude.
Previous Engine Power Events
The airplane had experienced several loss of engine power events prior to this incident. The first such event occurred shortly after the airplane was sold to a new buyer. An inspection of the fuel tanks revealed contamination in the form of "beads" within the tank. After having the tanks flushed, a second loss of power event occurred. In each previous instance, after the airplane landed, the engine would produce power again. Following the second event, the carburetor was examined and the airplane was returned to service. Approximately 15 to 30 hours later, another loss of power event happened. This time, the carburetor was sent to a repair facility, which rebuilt the carburetor and informed the owner that a "wrong jet" had been installed. The airplane then flew another 50 to 60 hours without a recurrence of the issue.
Fuel Cap Replacement
At some point prior to the accident, the airplane's original fuel caps were replaced. The installed caps were an aftermarket design, with the label "2646V" inked on the top. The vent holes in these caps were less than half the size of the original fuel caps. Additionally, it was possible for the gasket in the fuel cap to cover one or both of the vent holes. A review of the airplane's records did not reveal when the original fuel caps were replaced.
Investigation Outcome
Despite examination of the airplane and its systems, the reason for the reported loss of engine power could not be determined.