Incident Summary
An aerial application flight was returning to its base field after completing a spray run. The pilot reported that a puff of black smoke was emitted from the exhaust, and the engine subsequently quit. With dense woods below, the aircraft was forced to land in the trees.
Examination Findings
A post-incident examination of the engine revealed that two rivets that fastened the carburetor accelerator pump piston had broken. These rivets were found in the carburetor float chamber, with one of them located near the needle and seat assembly.
Probable Cause
The engine likely quit when one of the broken rivets became lodged under the needle, preventing it from seating properly, which allowed the carburetor to flood with fuel.