Event Description
During the takeoff initial climb, the engine lost power. The airplane subsequently collided with trees short of the intended forced landing area.
Examination and Testing
Examination of the airplane revealed that it was using automotive gasoline. During a subsequent test run, the engine operated at full rpm, then began to lose power output by 300 to 400 rpm. Carburetor heat was applied, and the engine gained partial return of power. When the carburetor heat was removed, the engine returned to the previous running condition.
Carburetor Issue
A float level problem was suspected, and the carburetor was sent out for analysis at an overhaul shop. However, the aircraft owner subsequently decided to replace the carburetor with a different type, and the accident carburetor was lost in the overhaul process.
Relevant Advisory
Information Letter 8401 (dated December 15, 1984) from the Experimental Aircraft Association (EAA) advises of a long standing problem with rubber tipped needle valves which swell when exposed to automotive fuels. The letter states that slow degeneration of in-flight engine power has been observed when the rubber needle valve tips swell and decrease the orifice size at the carburetor inlet. Sudden losses of power have also been reported during takeoffs in aircraft following a long period of inactivity.