Lean won't cause flame. Rich will. (Within reason, you can be so rich that your EGTs are really low and thus external combustion won't happen) The hot fuel vapors meeting cool oxygen = burny burn.
That said, you usually see it in turbo applications where there's a spike in exhaust fuel content on up-shift due to boost fall off and the exhaust is really hot due to being turbo (naturally higher EGT, turbine heating exhaust gasses a little immediately off boost).
Last edited by Sh0velMan; 06-05-2013 at 10:20 PM.
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