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Originally Posted by Jim H
When the air:fuel ratio is too 'lean' or too 'rich' the mixture cannot be ignited by a spark. You may be able to get the fuel to burn by applying some other ignition source, but not with a spark plug.
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I think it worthy of mention that an A/F mixture around 14.7:1 is also necessary for controlling exhaust emissions. A too-rich mixture will cause CO and HC to climb, while too-lean causes an increase in NOx. 14.7:1 represents the best compromise between power, economy, driveability, and emissions.