If the air is more relative to the fuel, the mixture is called a lean mixture. This results in lesser power generation. The required torque will not be developed.
If the air is less compared to the fuel, the mixture is called a rich mixture. This results in incomplete combustion. The smoke emitted is dark in color due to the presence of carbon particles which have not been oxidised. The engine will not develop the required power.
Stoichiometric combustion is the ideal combustion in which all the fuel is combusted with no excess air. It is not possible to mix the ideal amount of fuel with the ideal amount of gas in practice.
The ratio of the mass of air to the mass of fuel is called the Air-Fuel ratio or AFR.