When an engine goes boom

mgencleyn

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Ever heard the loud bang or boom from an old engine, like a lawn mower or tractor? What is that? What causes it?
 

alleycat

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It's called a backfire or misfire. It's generally caused when all the gas doesn't burn in the cylinder head like it's suppose to. Then it exploded in the exhaust system.

Edited to add: movieman beat me posting by a few seconds. We're saying the same thing.