What is a petcock and how do you use it?

Answer

Here's the answer from Timberwoof's Motorcycle FAQ: "It's a Main/Reserve fuel valve. Its purpose is to select which of two tubes the fuel pump uses to get fuel from the fuel tank. One of the tubes is at the bottom of the tank; the other is about an inch higher. The normal 'On' position gets fuel from the tube with the higher opening. When it runs dry, you switch to 'Reserve' which gets that last gallon or so from the bottom of the tank. That gives you enough range to make it to the next gas station for a fill-up."

Answer

Modern petcocks have no Off position. They have a vacuum valve that shuts off the gas when (for any reason) the engine stops. There are three positions of the petcock lever: Pri(me) which overrides the vacuum system to allow dry carburretors to be refilled; never leave the petcock in the Pri position after the carbs are full, carbs can flood, moreover the engine might not shut off if you drop the bike. The On and Res(erve) are held open by the vacuum from the intake manifold. They function as in the first answer. TIP. Fill the tank and set the milometer trip to zero. Check the range of a full tank less reserve. When the milometer trip reads 90% of the range (mine is 100 miles), refill the tank. You will never use reserve, or spend days getting crud out of the carbs again.

Tip for newer bikes: If you happen to be fortunate enough to have a newer bike it has been suggested to me to run your bike in reserve every few hundred miles to keep gunk from building up in your tank. This way you don't have to worry if you'll damage your bike if you have to use the reserve.

First answer by ID1369410650. Last edit by Ki Yote. Contributor trust: 14 [recommend contributor]. Question popularity: 138 [recommend question]