Whats a Petcock

Is a regulator consisting of a small cock or faucet or valve for letting out air or releasing compression or draining. Although petcocks are used in a wide variety of applications, the following passage will describe one of the most common applications of the petcock which is the control of gasoline on a motorcycle engine.

Most motorcycles have a fuel petcock valve mounted on or nearby the gas tank to control the supply of gasoline. The petcock typically has three positions: ON, OFF, and RESERVE. The reserve position accesses the bottom portion of the gas tank. The reserve position functionality of the petcock is especially useful on motorcycles because they often don't possess a fuel gauge.

When operating a motorcycle the fuel management process often proceeds as follows: Especially when regarding vintage motorcycles the petcock is set to the off position when the motorcycle is not being operated. This is to eliminate fuel overflow and leakage via the carburetor(s). Before starting the engine the petcock is turned to the ON position in order to provide gasoline to the fuel delivery system.

While operating the engine there will reach a point at which fuel consumption causes the level of gasoline in the gas tank to fall below that which can be accessed by the petcock in the ON position. At that time continued operation of the engine can be maintained. This operation is achieved by accessing the remaining fuel in the gas tank via rotating the valve in the petcock to the RESERVE position.