Motorcycle Fuel Petcocks

A motorcycle fuel petcock is a fuel valve which the rider can use to shut off or turn on the flow of gasoline from the fuel tank into the carburetor. The device is usually bolted to the bottom of the gas tank and connected to the rest of the fuel delivery system with a hose. A typical manual petcock utilizes a ball valve construction, which limits the use of the tap to simple “on” and “off” applications. Modern motorcycles utilize vacuum-operated fuel petcocks. The valve can be opened only when a vacuum is created in the system, which means that the fuel flows only when the engine is running.

Vacuum-operated or automatic petcocks have additional functions, such as “reserve” and “prime.” The former is widely used in motorcycles with small tanks not equipped with fuel gauges. To avoid situations when you have to push your bike to the nearest gas station because of the empty tank, engineers made the petcock collect gasoline via the tube enclosed into a fine mesh filter and raised above the bottom of the tank. When the rider feels that the engine starts choking as if the tank is empty, he or she can switch the petcock to the “reserve” mode to access fuel through another tube positioned closer to the bottom of the gas tank.

Automatic fuel valves don’t need a manual “off” option since they are always closed when the engine is not running. The “prime” mode is included solely for bleeding the system after servicing the carburetor or after running out of fuel completely. If you fill the tank, but the bike would not start, you can turn the petcock into the “prime” position to bypass the vacuum-operated valve. Fuel flows into the carburetor, and then you can start your motorcycle. After the engine starts, you need to switch the valve into the “on” position so that it could be operated by vacuum again. Fail to do that, and you can flood your carb and motor with gasoline potentially causing damage.

If you notice that your petcock is leaking fuel, it means that you need to rebuild the assembly as soon as possible to prevent gasoline loss and, more importantly, potential fire. In our store, you can find a wide range of fuel petcock repair kits that include all of the necessary O-rings, gaskets, and diaphragms. First of all, you need to drain remaining fuel, remove the fuel and vacuum lines secured with clamps from the fuel tap. If you want to avoid breaking the seal between the tank and the petcock, you have to remove your tank to work on your fuel valve. You might need to take the seat off to access bolts holding the gas tank.

Be careful not to lose the spring pushing on the vacuum-operated diaphragm valve when unscrewing the vacuum section of the petcock. Replace the diaphragm. Make sure to install an O-ring on the valve itself. When tightening screws on the cover, do it in a criss-cross pattern. To remove the cut-off valve cover, you need to undo two screws. A special ring spring keeps the valve firmly seated in its housing. Remove the valve to access the rubber seal. Over time, this seal can get worn and compacted because of the constant pressure. Replace all damaged components and reassemble component in the reverse order.