Mercedes Mullins
A fuel petcock is a small shut-off valve used to control the flow of liquid or gas. Older motorcycles typically have a three-position fuel petcock valve (known as a petrol tap in the UK) mounted on or near the fuel tank to control the supply of gasoline: on, off, and reserve. The reserve position accesses the bottom portion of the fuel tank. Many older motorcycles lacked a fuel gauge, making reliance on a fuel reserve a common experience. Modern petcocks are typically automatic and vacuum-operated, and may include a prime position which allows fuel to flow to the carburetor without the engine turning over.
| Characteristics | Values |
|---|---|
| Definition | A small shut-off valve used to control the flow of liquid or gas |
| Use | To control the supply of gasoline in vehicles |
| Types | Ball valve style, Gate valve style, Vacuum petcock |
| Parts | Diaphragm, Spring, Float needle |
| Common Applications | Older motorcycles, Automobiles as a drain valve for a radiator, Vehicle heating and cooling systems |
| Price Range | $5.99 - $152.95 |
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What You'll Learn
How does a vacuum petcock work?
A vacuum petcock is a type of fuel petcock that is commonly found on motorcycles. It is designed to control the flow of fuel to the engine, ensuring that the engine only receives fuel when it is running.
The vacuum petcock works using a diaphragm and a spring. When the engine is running, the vacuum created by the engine pulls the diaphragm back, compressing the spring and releasing pressure on a ball or valve. This allows fuel to flow around the ball and towards the carburettor.
The “prime” position on the petcock allows the carburettor float bowl to fill with fuel, even when the engine is not running. This is useful if the engine has run out of fuel or the fuel line has been drained for maintenance. Once the engine is started, the vacuum created will allow the petcock to switch from "prime" to "on" or "off".
Vacuum petcocks can fail if the diaphragm splits or the vacuum line becomes disconnected or damaged. This can cause fuel to leak and the engine to flood.
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How to switch the petcock while riding
A petcock is a small shut-off valve used to control the flow of liquid or gas. Older motorcycles typically have a three-position fuel petcock valve (known as a "petrol tap" in the UK) mounted on or near the fuel tank to control the supply of gasoline: on, off, and reserve.
Step 1: Understand the Basics
Know the three positions of the petcock: "On," "Reserve," and "Off." The "On" position is used when the fuel tank is between one-quarter full and completely full. The "Reserve" position is used when the fuel level drops below the "On" level, and the bike starts to stumble. The "Off" position shuts off the fuel flow, useful when you're not riding or when you need to drain the fuel system for maintenance.
Step 2: Monitor Your Fuel Level
Keep an eye on your fuel gauge or trip meter. When the fuel level drops below the "On" tube level, you'll feel the bike start to stumble. This is your cue to switch to the "Reserve" position.
Step 3: Switch to Reserve
When the bike starts to stumble, quickly switch the petcock to the "Reserve" position. This will start the fuel flowing through the shorter "Reserve" tube, allowing you to draw from the remaining fuel in the bottom portion of the tank.
Step 4: Refuel and Switch Back to "On"
After switching to "Reserve," you have a limited amount of fuel, so start looking for a place to refuel as soon as possible. Once you fill up, remember to switch the petcock back to the "On" position, and you can continue riding.
Step 5: Preventative Measures
To avoid running out of fuel unexpectedly, get into the habit of turning the petcock to "Off" about 30 seconds before turning off the engine. This will ensure that any remaining fuel in the system is used up. Additionally, always set your trip meter when you fill up, so you can anticipate when you might need to switch to "Reserve."
Step 6: Troubleshooting
If your bike has carburetor issues or tends to flood, consider turning the petcock to "Off" when you're done riding. This will prevent fuel leakage and potential engine damage.
By following these steps, you can effectively manage your fuel supply while riding and avoid the dangers of running out of fuel in unsafe locations. Remember to pay attention to your fuel levels and be mindful of the petcock's different positions to ensure a smooth and enjoyable riding experience.
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The origin of the term 'petcock'
A petcock is a small shut-off valve used to control the flow of liquid or gas. The term 'petcock' is derived from the cock of a firearm, which when released by the action of the trigger, discharges the piece. The cock of a firearm is first recorded as being used in the 1560s, with the term 'half-cocked' also coming into use around this time.
Historically, petcocks were threaded valves controlled by a butterfly handle, whereas modern petcocks are typically ball valves. Ball valve-style petcocks are used principally for on-off applications. Older motorcycles often have a three-position fuel petcock valve (known as a "petrol tap" in the UK) mounted on or near the fuel tank to control the supply of gasoline: on, off, and reserve. The reserve position accesses the bottom portion of the fuel tank, which was particularly useful for older motorcycles that lacked a fuel gauge.
Petcocks are still used in vehicle heating and cooling systems, such as limousines and tractor trailers. They are also used to periodically drain water from diesel fuel water separators on some trucks.
In addition to their use in vehicles, petcocks are also used in pressure canners, where they are one of the many components that make up the hulking and heavy design of these devices.
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$9.99
The function of a fuel petcock
A petcock is a small shut-off valve used to control the flow of liquid or gas. They are typically used in engines and heating and cooling systems.
In engines, a petcock is used to control the supply of gasoline to the engine. They are most commonly found in older motorcycles, where they are typically mounted on or near the fuel tank. The three-position fuel petcock valve has 'on', 'off', and 'reserve' settings. When the petcock is in the 'on' position, it allows access to most of the tank, with a little left in reserve. When the fuel in the tank drops below a certain point, the engine will sputter and start to die. At this point, the rider can switch the petcock to the 'reserve' setting, which accesses the bottom portion of the fuel tank. This allows the rider to get to a gas station to refuel. The 'off' position is a precaution to prevent fuel from leaking into the cylinders and sump, which can cause damage to the engine.
Petcocks are also used in vehicle heating and cooling systems, such as limousines and tractor trailers. For example, petcocks are used to periodically drain water from diesel fuel water separators on some trucks.
In addition, petcocks were historically used on gravity-feed fuel systems found in automobiles such as the Porsche 356 and early Volkswagens.
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The difference between a ball valve and a gate valve
A petcock is a small shut-off valve used to control the flow of liquid or gas. Historically, petcocks were threaded valves controlled by a butterfly handle; modern petcocks are typically ball valves.
Ball valves and gate valves are two of the most commonly used valves for controlling the flow of fluid in plumbing or gas systems. They have similarities in application but differ in several ways.
Ball Valves
Ball valves are also called shut-off valves and are commonly found in newer homes. They use a handle to control the placement of a hollow, perforated sphere in the mechanism. When the handle is turned, the hole immediately opens or closes the water flow. This mechanism is a lot quicker than a gate valve because a ball valve needs only a 90-degree turn to go from open to completely closed. Ball valves are ideal for applications that require fast cycle times, multiple ports, reliable seals, and frequent operation. They are commonly used in the automotive industry, manufacturing facilities, and food and beverage equipment.
Gate Valves
Gate valves, also called sluice valves, are more likely to be found in older houses. Their mechanism usually involves a knob that, when turned, lowers or raises an internal gate or wedge to control the water flow rate. Gate valves allow you to control the rate of the flow of water but don't provide an immediate stop. They are usually controlled by a manual handwheel that requires more than a 360-degree turn to change the gate's position. Gate valves are best suited for applications that require infrequent operation and small installation spaces. They are commonly used in large water supply lines, mining, and power plants.
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Frequently asked questions
A fuel petcock is a small shut-off valve used to control the flow of liquid or gas.
A fuel petcock has three positions: on, off, and reserve. When the lever is turned to on, gas flows through the taller tube. When the lever is turned to off, no gas flows. The reserve position accesses the bottom portion of the fuel tank.
When the fuel in the tank drops below a certain point, the engine will sputter and start to die. You can switch to the reserve setting to access the remainder of the tank, which should be enough to get you to a gas station.
The price of a fuel petcock depends on the type and quality. Fuel petcocks are available for as little as $9.45, while others can cost over $100.
