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In motorcycles, when the main fuel runs out, the rider has to manually turn the reserve knob, usually located in the left part of the engine, to the reserve position. This allows the rider to use the remaining fuel.
The purpose of the fuel reserve is to serve as a warning to the rider that fuel is low, as many motorcycles do not have fuel gauges and determining the fuel remaining is difficult. 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 small quantity of fuel. Often when a rider notices the motor start to sputter, they can access the reserve before the engine shuts down.
In most cases, the main tank and the reserve tank are actually 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 not set to the reserve position, the fuel will flow from the upper hole, and will stop flowing when the fuel level gets below the upper hole. When the selector is on reserve, the lower hole 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 they may run out of fuel without warning.