Backfire

[1] Passage of unburned fuel mixture into the exhaust system where it is ignited and causes an explosion (backfire) prematurely.
[2] Sometimes ignition takes place in the intake manifold by a flame from a cylinder because the intake valve leaks. Burning of the fuel mixture in the intake manifold may be caused by faulty timing, crossed plug wires, leaky intake valve, etc.
[3] A welding term referring to a short "pop" of the torch flame followed by extinguishing of the flame or continued burning of the gasses.