Petcock. It's an odd name isn't?
Whoever came up with it must have lived prior to the invention of motion pictures. Or had a chicken farm. At any rate, the petcock, or "fuel petcock" is a valve on non-fuel-injected 2-stroke and 4-stroke dirt bikes that controls the flow of, you guessed it, fuel.
Fuel Petcock Positions
In a nutshell, if it's turned "on" you can ride, if it's "off" you can't ride. A third position which your dirt bike might have is the "reserve" which provides access to the bottom portion of the fuel tank or what's considered the reserve. It's basically enough to get back to camp. Another position option is "prime" which allows fuel to flow without a vacuum like the "on" position provides, and gives fuel access to the carburetor without turning the engine on. However, operating your bike in "prime" for too long causes the carburetor to overflow.
Very few petcocks offer all four positions. Some "prime" positions tap into the reserve negating the need for a "reserve" position. Those without "reserve" don't have a reserve. You might even find a petcock without an "off" position. It's all very confusing so to sum up: Most, if not all dirt bikes, with a petcock have at a minimum an "on" and "off" while others incorporate "prime" and/or "reserve" positions.
A fuel petcock with "reserve" position. The longer straw is the feed, the shorter taps into the reserve.
Well, why even have an "off" position? That's so you can perform some general maintenance. If you stop the flow of fuel to the carb you can remove the carb, if necessary, without thoroughly pinching off fuel line hoses. Additionally, you'll want to turn the petcock "off" when encountering a stuck float. Finally, when garage bound it's always best to have it "off" in the event of a tip over or a leak. Imagine going on vacation after leaving the petcock "on." Fuel leaks from the vent of the carburetor, the natural gas furnace in the garage kicks on. Blam!
Leaking Petcock Symptoms
Sometimes the petcock leaks. That's why you're here. You can ride but probably not very far and whenever gasoline is around a hot engine, it's probably not such a good idea. So fix it. It's pretty simple and all you need is a repair kit.
A leaky petcock won't give you many fits while riding other than drip fuel out of the flood tube and end your day faster. At worst, fuel fills the cylinder, eliminates compression and prevents the piston from moving. Once the petcock is "off" you'll notice gas leaking out of the face of the petcock. That's when you know it's the petcock and not anything else associated with the fuel system.
How to Fix a Leaking Petcock
This is probably one of the more relaxed repair jobs on your dirt bike. First it's best to drain the fuel but in some cases you can plug the fuel tank cap vent line and turn the tank over if little fuel remains. Now grab a repair kit. Remove the screws located on the valve (what you use to switch to the "on," and "off" positions), remove the petcock and replace all the gaskets, bolts and screws. This is the simplest repair to most leaky fuel petcocks.
The alternative is replacing the old petcock with a new one in the event of a crash, other damage or age compromising the integrity of the petcock, or if replacement parts don't exist for the old one. This isn't too hard either as the petcock generally slides out and quickly replaced by sliding the new one in and attaching using the supplied bolts. Additionally, many owners find the repair kit doesn't always solve the leak problem and end up replacing the old one.
However, if you're running an Acerbis fuel tank you can only use an Acerbis petcock, the same with Clarke. All others head to OEM.
In many respects a replacement job is probably easier than a repair job. It's basically removing the old one and installing the new one. Some modifications to the gas tank might be necessary in the event the new petcock doesn't line up perfectly with the bolt holes but outside of that it's fairly straightforward.
If you discover your leaky gas problem is not the petcock then check out these other resources:
