In this month’s tech talk, Carl gives an overview of fuel petcocks and walks through the process of rebuilding one. Fuel petcocks sit between the tank and the carburetor, and are typically either vacuum-fed or gravity-fed.
Vacuum feed is standard on most OEM fuel tanks, and require vacuum (from the motor) to open a diaphragm that allows fuel to flow into the carburetor. These petcocks typically have three positions: on, reserve, and prime. The on position picks up fuel from a bit higher in the tank, so as to provide a warning when you start running low on fuel, while reserve picks up fuel from the bottom of the tank. Both the on and reserve positions require the motor to be running to open the diaphragm and allow fuel to flow. The prime position will always flow fuel, and is used to prime a dry motor and carb. Do not leave the fuel petcock in prime when the engine is not running as this could flood the motor or airbox.
A gravity feed petcock does not require vacuum to function. These fuel petcocks have three positions: on, reserve, and off. In on or reserve, fuel will flow whether the motor is running or not. If you park your motorcycle for more than a couple of days, be sure to switch the petcock to the off position.
Rebuilding a petcock is a simple process, but you’ll need to be particularly delicate with a vacuum-fed unit. While taking apart the fuel petcock, be sure to hold onto the backing plate when removing the screws. There is a spring behind this plate that could go flying once the last screw is removed. Another thing to be mindful of is the position of the diaphragm plunger, as this only goes in one way. Pay attention during disassembly so that you’ll remember how to reinstall it correctly. The diaphragm material can be fragile, so handle it gently. Lastly, you’ll need to line up the pins on the edges of the diaphragm plate to the holes on the new diaphragm for it to function properly.
On the other hand, a gravity-fed fuel petcock is very easy to rebuild. You’ll simply need to replace a few o-rings that have worn away.
As always, if you have any questions, reach out to us, and we’ll be happy to help!