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"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich, and you have unburned fuel in the exhaust system; cross firing from one spark plug wire to another can occur. If they are touching each other and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang. Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.

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Q: What causes backfire when starting engine?
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