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In motorcycles and cars, 'fuel reserve uses less fuel by using a smaller opening in the fuel valve.
When the main fuel is exhausted, the motor will stop running, prompting the rider to change the position knob to continue riding with a known smaller quantity of fuel. Often when a rider notices the motor start to sputter, he or she can access the reserve before the engine shuts down.
In most cases, the main and the reserve settings are actually drawing from the same tank, but there are two outlets through which the fuel may leave. One outlet is located a short distance above the other, when the fuel selector is set to the 'main' position, the fuel will flow from the upper outlet, and will stop flowing when the fuel level gets below the outlet. When the selector is on 'reserve', the lower outlet will be used, which allows all the fuel to be drawn from the tank.
Because in most cases the 'reserve' setting simply allows access to all the fuel, rather than a dedicated tank, the 'reserve' selection allows the rider to deplete the main fuel supply as well as the reserve. This causes no harm, except that the fuel may run out without warning.