Narrative:

The aircraft had sat on the ramp for about 5 days; where the last 3 days included some rain. During pre-flight; I noticed a small fuel stain on the bottom of the left engine firewall (inboard side of nacelle). I also noted that the left main tank was dripping from the the overflow vent; having just been filled.I taxied to the runway and completed the engine run-up checks at the runway hold short line. When pulling the throttles back to idle after the run-up I heard a sound that was later determined to be fuel vapor igniting. At the time I didn't know what it was. My passenger informed me that the left wall panel had been blown out towards his leg and we saw smoke from the vent on the center wing spar cover.at this point I notified the tower of the smoke. Immediately after that I saw flames through the spar cover openings and [advised] the tower. I requested fire trucks; and communicated souls; fuel on-board; and the intent to evacuate. I shut down all fuel and electrical systems. As I exited the aircraft; the flame had stopped. I believe that shutting the engines down and turning off the fuel selectors stopped the fuel from feeding the fire.I informed emergency personnel that I believed the flames were out after I turned everything off. They verified that the fire was out and we were towed back to the ramp and the aircraft was put into maintenance to determine the issue.I believe that fuel leaked from the left side of the aircraft; I don't know where from. This fuel must have been ignited during the run-up with hot engines.finding and fixing fuel leaks might have helped prevent this situation.

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Original NASA ASRS Text

Title: The pilot of a PA31 observed smoke and a small fire during run-up. An aircraft shutdown and evacuation were executed. A fuel leak from an engine driven fuel pump is suspected.

Narrative: The aircraft had sat on the ramp for about 5 days; where the last 3 days included some rain. During pre-flight; I noticed a small fuel stain on the bottom of the left engine firewall (inboard side of nacelle). I also noted that the left main tank was dripping from the the overflow vent; having just been filled.I taxied to the runway and completed the engine run-up checks at the runway hold short line. When pulling the throttles back to idle after the run-up I heard a sound that was later determined to be fuel vapor igniting. At the time I didn't know what it was. My passenger informed me that the left wall panel had been blown out towards his leg and we saw smoke from the vent on the center wing spar cover.At this point I notified the Tower of the smoke. Immediately after that I saw flames through the spar cover openings and [advised] the tower. I requested fire trucks; and communicated souls; fuel on-board; and the intent to evacuate. I shut down all fuel and electrical systems. As I exited the aircraft; the flame had stopped. I believe that shutting the engines down and turning off the fuel selectors stopped the fuel from feeding the fire.I informed emergency personnel that I believed the flames were out after I turned everything off. They verified that the fire was out and we were towed back to the ramp and the aircraft was put into maintenance to determine the issue.I believe that fuel leaked from the left side of the aircraft; I don't know where from. This fuel must have been ignited during the run-up with hot engines.Finding and fixing fuel leaks might have helped prevent this situation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.