While attending a fly-in this weekend, a number of us witnessed a Piper Warrior taxi out, do a rather long runup (with some backfiring during the mag check by selecting OFF). Whether it was a bad mag or leaded/fouled plugs, I don't know, but I suspect fouled plugs.
Rather than takeoff, the pilot, with instructor in the right seat, taxies back, shuts down and confers with someone rather experienced to determine what to do. The pilot and instructor re-enter the Warrior and attempt a hot start (Lycoming, carbureted four-banger). A fire in the cowling ensues after repeated cranking. Even though I am 100 yds away, I am the first to arrive with a fire extinguisher and successfully extinguish the fire after running by zombie on lookers.
You can imagine all the why's and how's like me, but I feel it all started by taxiing out with mixture full rich, causing the plugs to foul, which resulted in the excessive mag drop and the taxi back. The CFI didn't know to lean during taxi, how to clear the plugs during the runup, nor how to do a successful hot start.
shame all around.
Learn from this...and carry a fire extinguisher.
FYI...