What happened
During the initial climb following takeoff, the engine of the aircraft lost power. This loss of power caused the airplane to collide with trees located short of the pilot's intended forced landing area.
The investigation
An examination of the aircraft revealed that automotive gasoline was being used as fuel. During a previous test run, the engine had operated at full RPM before experiencing a power output drop of 300 to 400 RPM. In that instance, applying carburetor heat resulted in a partial return of power, but removing the heat caused the engine to return to its degraded running condition.
While a float level problem was suspected and the carburetor was sent to an overhaul shop for analysis, the aircraft owner decided to replace the unit with a different type. Consequently, the specific carburetor involved in the accident was lost during the overhaul process.
Findings
Information Letter 8401 from the EAA notes a known issue regarding rubber tipped needle valves that swell when they come into contact with automotive fuels. This swelling can decrease the orifice size at the carburetor inlet, leading to a slow degeneration of in-flight engine power. Additionally, sudden power losses during takeoff have been reported in aircraft following extended periods of inactivity.