Stoichiometric is the term given to an air to fuel ratio which gives the theoretically perfect balance of air and fuel.
In a petrol engine, the stoichiometric ratio is approximately 14.7 parts air to one part fuel (by weight).
If the air to fuel ratio is greater than the stoichiometric value, ie there is an excess of air, the mixture is lean. If the air to fuel ratio is less than the stoichiometric value, ie there is excess fuel, the mixture is rich.
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