Maintenance of the Fuel Cock also Affects the Engine Condition, So Be Careful Not Only the Outer Leaks But Also the Inner Leaks

How to

Even though there is a float valve in the carburetor, it is the important role of the fuel cock to stop the fuel that is about to fall by gravity without fail. If the rubber parts built into the fuel cock deteriorate, it can cause various problems, so it is necessary to take appropriate action according to the nature of the problem.

They are classified as negative pressure fuel cocks and gravity fuel cocks based on the difference in the way fuel is fed.

The fuel cock on the Kawasaki GPZ400F is a negative pressure type with three positions: ON, RES, and PRI. In the ON and PRI positions, when the engine is started and the diaphragm of the fuel cock is subjected to the negative pressure of the manifold, the passage is opened and fuel flows from the tank to the carburetor. In the case of gravity-type cocks of the Z1/Z2 era, if the fuel cocks are left ON or RES instead of OFF when parking, the fuel will continue to flow from the tank to the carburetor, increasing the risk of overflow.

Fuel cock is indispensable for fuel system of fuel injection carbureted motorcycle. Before the negative pressure type cock that the passage is opened by the negative pressure of the manifold of the engine became popular, three positions of ON-OFF-RES were common in the gravity type cock, and three positions of ON-RES-PRI was seen in the negative pressure type cock.

When you leave the motorcycle with the lever on the gravity-type cock, the fuel in the tank is constantly trying to flow into the carburetor, so the float in the float chamber and the float valve are constantly under load. Therefore, it was not uncommon to stop the motorcycle at the side stand and cause the oil level to tilt, on which the valve would not close completely, causing an overflow. Therefore, with the gravity-type cock, it was mandatory to turn off the fuel cock when getting off the motorcycle. The introduction of the negative pressure fuel cock, which did not require the fuel cock to be turned off even after getting off the motorcycle, was a revolutionary event.

In both fuel cocks, the internal passage is switched by changing the position of the lever to guide the fuel from the tank to the carburetor. The internal rubber parts are important to achieve both smooth lever movement and fuel sealing. It is an important task for a manufacturer to develop a fuel cock that shows stable performance over a long period of time by using a material that does not swell when soaked in fuel and does not shrink when the fuel is gone and dried.

For the manufacturers who develop fuel cocks, it is true that the products manufactured based on decades of experience are durable and reliable enough. However, it is also true that not all users will be satisfied. There are real cases that the lever movement became extremely bad with the fuel cock after a long time from the new motorcycle, or the paint swelled up due to the fuel seeping at the mounting surface of the tank and the fuel cock. Have you ever disassembled a switching lever and found that the rubber seal inside had swollen and was difficult to restore or could not be put back together? On the other hand, there are also cases where the rubber has lost weight and the sealing performance has deteriorated, causing a problem.

In both cases, the cause is often the age-related deterioration of the rubber material. Swollen and stuck levers or thin and oozing or leaking are both problems, but it is the changes caused by shrinkage that need to be addressed in terms of direct danger.

POINT
  • Point 1: There are two types of fuel cocks for carbureted vehicles: negative pressure type and gravity type.
  • Point 2: Deterioration of the rubber parts that ensure internal sealing causes fuel seepage and leakage.

The condition of the gasket on the joint surface between the fuel tank and the fuel cock is super important.

When cleaning the fuel cock, remove any debris left in the gasket groove at the tank mounting. Be especially careful if a liquid gasket has been applied to stop seepage or leakage from the mating surfaces. There are two pipes inserted into the tank, the longer one leads to ON and the shorter one leads to OFF. On relatively newer motorcycles, both the pipe and strainer are made of plastic, but on older motorcycles, there is a combination of brass pipe and fine wire mesh.

If the oval-shaped gasket between the fuel cock and fuel tank is still functioning, the paint on the back of the tank will not lift or peel off. If fuel seeps into the mating area, or if the paint film floats when the fuel cock is removed, replace the gasket with a new one and use a scraper to remove the paint film from the contact area.

One of the most common fuel cock problems on out-of-print models is leakage or seepage from the gasket on the mounting surface to the fuel tank. In some cases, the paint on the boundary between the fuel cock and the gasket becomes soggy, the gasket turns brown due to fuel penetration, or the bolts that hold the fuel cock in place or the area around the fuel hose that is inserted become damp. When such a situation is found, unless the tightening torque has been released, simply retightening the mounting bolts and screws will not solve the problem, but may trigger further deterioration.

If the gasket is worn out due to aging and the sealing property is deteriorated, the seepage will not stop even if the bolt is tightened because the fuel cock and the tank are in perfect contact. On the contrary, there is a risk that the fuel cock itself will be deformed by over-torque and the leakage will increase. In the case of a carburetor that is leaking fuel from the mating surface of the carburetor and the float chamber due to gasket loss, tightening the bolts will distort the body and make the leak worse.

The only way to deal with this problem is to replace the gasket with a new one. Sometimes a fuel-resistant liquid gasket can be applied to stop the leak, but a new gasket is not only thicker and more flexible, but it also has a linear rather than a solid contact surface with the tank and fuel cock, which increases surface pressure and improves sealing. This is why it is fundamentally different from a liquid gasket applied to a crushed gasket. However, it is effective to apply it as a supplement to a new gasket.

When fitting a fuel cock with a new gasket to the tank, it is important to remove any dirt from the seating surface. Especially if the painted surface is peeling off in some places due to fuel immersion, the paint film on the part of the fuel cock where the gasket contacts should be cleaned off with a scraper before installation. Although peeling off the paint will make you worry about rust prevention, it is safer than installing the fuel cock on the stepped paint film.

POINT
  • Point 1: If the gasket between the fuel cock and the fuel tank has deteriorated over time, replace it with a new one.

Defective O-rings in the diaphragm can also cause leakage.

A thin wave washer that pushes the lever to the packing side is incorporated between the panel and the lever to fix the switching lever. When fuel oozes from around the lever due to deterioration of the packing, it is sometimes necessary to retighten the small screw to fix the panel. However, if the panel is in contact with the fuel cock body, it is useless to tighten the screw further. However, if the panel is in contact with the fuel cock body, tightening the screws will not help. On the contrary, it will cause distortion of the fuel cock and panel.

An O-ring is incorporated around the periphery of the fuel cock to prevent fuel leaks, along with a four-piece perforated rubber packing. Wetting the O-ring with fuel creates a moderate amount of slippage, but if it is completely dry, the rubber may bite and twist, or the surface may become wispy, causing fuel leakage.

The most common failure inside the fuel cock is the deterioration of sealing due to the effect of the switching gasket on the backside of the lever. The gasket, which is a thin disc-shaped plate with several holes, is called "Lincoln Packing" because of its shape. It switches the ON-RES-PRI fuel passage depending on the position of the lever, and the lever slides while being pressed against the gasket. If there is fuel between the packing and the lever, it itself acts as a lubricant and moves smoothly, but if there is a period of time when the vehicle is not ridden for a while and the packing dries out, movement may become difficult.

If you force the lever to move at this point, it will strain the gasket and cause it to wear, so if you start the engine and let the fuel through once, it may perform smoothly after that. If the problem persists, remove the fuel cock from the tank, disassemble the lever, and apply internal silicon grease to restore it. If hardening due to aging is suspected, it is safe to replace the drain packing.

In the case of a negative pressure-type cock, another important thing is the O-ring attached to the diaphragm.It is the presence of the A plunger is attached to the center of the diaphragm by the negative pressure generated by the engine, which opens and closes the fuel passage in front of the switching lever, and a small O-ring is incorporated in the sealing part. In the image, the fuel cock for Kawasaki GPZ400F is used as a sample, but Zephyr and other models have the same structure. The O-ring at the tip of this plunger operates with a very small stroke in accordance with the diaphragm operation. Therefore, it is not likely to be damaged under normal circumstances.

However, if rust generated in the fuel tank flows into the fuel cock and gets caught in the O-ring, it can cause problems. The fuel cock has pipes of different heights for ON and RES in the tank, and a strainer is mounted at the end of the pipe to prevent foreign matter from entering. So there's no way for foreign objects to get into the fuel cock?However, it is not uncommon for rust particles to pass through the fuel cock and accumulate in the float chamber. In view of this reality, it is possible for foreign matter to remain in the fuel cock. Particularly when rust in the tank has progressed to some extent due to long-term neglect, red rust may also adhere to the fuel cock and inside the fuel hose, and it is often the case that even if the front side of the fuel cock, the lever, and the drain packing, is cleaned, the diaphragm side is missed.

At this time, if a piece of rust is caught in the O-ring of the plunger and the passage cannot be closed, the passage of fuel may become open even if the plunger is not open. If the plunger is not open, the passage of fuel will be open. If the plunger is open, the valve in the float chamber can stop the passage of fuel.

In the case of GPZ and Zephyr, the diaphragm itself is included in the fuel cock assembly and is not available as a separate part. However, the O-ring at the end of the plunger is a stand-alone part, so if fuel is flowing from the fuel cock at the ON or RES lever position without starting the engine, it may be a good idea to check the O-ring in this area.

In most cases, the diaphragm of the negative pressure type cock is located on the backside of the fuel cock, but in rare cases, there are models where only the diaphragm is set in a separate part from the fuel cock. By inserting the hose into the nipple sticking out to the left and inserting the other side into the intake manifold, the diaphragm is activated by the intake negative pressure.

Between the cover and the diaphragm, which is a thin film of rubber, there is a spring incorporated that applies a force to close the fuel passage. Thanks to this spring, the fuel is shut off when the engine is stopped. For this reason, if the diaphragm is damaged and does not operate under negative pressure, the engine cannot be started.

When the thin and delicate diaphragm is removed, there is an aluminum plunger with an O-ring attached to the tip on the backside (fuel passage side). When the plunger strokes, the O-ring opens and closes the fuel passage, so there is a danger of fuel constantly flowing if a foreign object gets caught in the O-ring. To avoid this, the two pipes of the fuel cock are equipped with mesh strainers, but if fine rust from the tank accumulates on the O-ring after a long period of inactivity, it may affect the sealing performance.

When replacing the gasket or O-ring, it is recommended to apply a thin layer of silicon grease to prevent biting before setting. Replacing the aged and tacky Lincoln gasket with a new one that is resilient will improve the lever movement by itself.

POINT
  • Point 1: If the Lincoln packing on the backside of the lever hardens, even if there is no obvious wear, the sealing property will deteriorate and cause leakage.
  • Point 2: Check the O-ring on the plunger tip of the diaphragm because it is easy to overlook.
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