Petcock

A petcock is a small valve used to control the flow of liquid or gas. 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.

Use on motorcycles

Most motorcycles have a fuel petcock valve mounted on or nearby the petrol 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 petrol tank. Its functionality is especially useful on older or more basic motorcycles, which may not possess a fuel gauge. Many motorcycles now have an automatic, vacuum operated, petcock, with on and reserve as well as a prime position, which is used if the fuel level is run so low that the engine cannot be started.

When operating a motorcycle the fuel management process often proceeds as follows: 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. 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 petrol 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 petrol tank via rotating the valve in the petcock to the reserve position.

Use on automobiles

Few, if any, automobiles currently in use have a petcock. This is due to the fact that fuel injection is prevalent on most engines as well as few cars, if any, having used gravity feed fuel supplies for many years. The Porsche 356 has a petcock with off, on, and reserve positions, much like a motorcycle petcock.

Limousines may use a petcock to control fuel flow, since fuel travels several feet farther than on typical automobiles. However, electronic petcocks are available on most prefabricated limousines.

Many automotive radiators use a petcock to allow for the draining of antifreeze, but this bears very little resemblance to fuel supply type petcocks.

References in Pop Culture

On the May 21, 2007 edition of The Howard Stern Show, Ronnie "The Limo Driver" Mund claimed that a petcock was the reason Howard's limo was cold in an argument about the inconsistent temperature of the limo.