At cruising speed, the engine is operating at a relatively low load, and so it does not need as much fuel to maintain its speed. This allows the engine to run at a leaner A/F ratio, which helps to improve fuel economy. However, if the A/F ratio becomes too lean, the engine will start to run rough and may eventually stall.
On the other hand, if the A/F ratio becomes too rich, the engine will start to use more fuel and produce more emissions. This is because the excess fuel will not be completely burned, and it will instead be emitted from the exhaust pipe.
Therefore, it is important to keep the A/F ratio as close to the stoichiometric ratio as possible in order to ensure optimal performance and fuel economy.
