What ignites the exhaust fumes on race cars to make flames?

race cars run a very rich mixture. when the engine dies, the unburnt fuel in the exhaust system catches fire from the heat in the exhaust pipes. when the engine is refired, the fire blows out. Flames come out the exhaust during the deceleration of a racing car because during wide open throttle the intake manifold and intake port are filled with a homogenized air fuel mixture. When the throttle is suddenly closed the velocity of this mixture stops and the pressure drops into a strong vacuum. This causes the fuel to drop out of suspension (in the air) and return to a liquid state. This liquid fuel is drawn through the combustion chamber and ignites when it reaches the hot exhaust valve and pipes resulting in the visable flames from the end of the pipe.