Background
According to the source, the accident sequence began after the aircraft was refueled. The pilot then attempted to start the engine.
Engine Start Attempt
During the start attempt, the throttle was positioned at full open, and the mixture was set to idle cut-off. The instructor assessed that the engine was suffering from vapor lock and planned to drain the gascolator. However, before doing so, he observed a fire in the carburetor airbox.
Firefighting Efforts
In response, the instructor first attempted to extinguish the fire by cranking the engine with the fuel turned off. This did not succeed. He then deployed a portable fire extinguisher. The fire did not spread from its initial location but was not extinguished because of the difficulty of reaching the area with the extinguisher.
Spread and Final Suppression
Once the portable extinguisher was exhausted, the fire spread to the interior of the aircraft. The North Andover Fire Department responded and ultimately extinguished the fire. The report notes that Lawrence Municipal Airport did not have firefighting equipment available at the time.