Explanation of the Structure of the Negative Pressure Fuel Cock and the Essentials of its Maintenance

The fuel cock is located between the fuel tank and the carburetor and switches the flow of gasoline. It is a part that is commonly operated on carbureted motorcycles, but some riders may not be familiar with its structure. Here we will disassemble the negative pressure cock and explain the causes of its malfunction.

ON and RES switch because of the different heights of the pipes in the fuel tank.

Fuel cock for Kawasaki Zephyr. The long left pipe at the top of the cock is for ON and the short right pipe is for RES. Both have a plastic mesh screen in the suction area to catch debris in the tank. The pipe on the RES side is short enough to suck gasoline from the bottom of the tank, so it can easily clog if there is dirt or rust on the bottom of the tank. The black, home-base-shaped part on the right side of the main unit is the thin-film diaphragm. It can be removed from the cock body with four screws, but it is not set as a diaphragm part.

After a long period of neglect, the inside of the tank rusted and the rubber packing, designed to look like a face, hardened, causing gasoline to seep through the outer circumference of the lever. When the inside of the tank is empty and the packing dries out, it shrinks, causing leakage the next time gasoline is poured into the tank. Conversely, if the rubber parts swell due to excessive soaking in gasoline, they may return to their original size by washing them with a mild detergent and drying them in the sun.

Fuel cocks, which are always equipped on carbureted sports models from mopeds to big bikes, are classified into two types according to the mechanism of operation: gravity type and negative pressure type. The gravity type is a simple switching cock that allows gasoline to flow whenever the lever is moved from OFF to ON or RES. In other words, even if the engine is stopped, gasoline will try to flow from the tank to the carburetor if the lever is ON.

If the carburetor float is functioning properly, the float valve will close when the oil level in the float chamber reaches a certain height, shutting off the flow of the fuel. However, if for some reason the float valve does not close, or if there is a gap, gasoline will continue to flow, causing an overflow, which is very dangerous. For this reason, gravity-cocked motorcycles must be turned off when parked or stored.

In contrast, the flow of gasoline in a negative-pressure cock depends on the condition of the engine in addition to lever switching. Negative pressure in the negative pressure type refers to the intake negative pressure generated in the intake port of the engine. Whether the lever position is ON or RES when the engine is started, negative pressure is generated in the intake port and the passage in the cock opens, allowing gasoline to flow from the tank to the carburetor.

On the flip side, if the engine is stopped, whether the cock is ON or RES, the passageway will not open and fuel will not flow into the carburetor and there is no risk of overflow while the vehicle is stopped.

So when the engine stalls due to out-of-gas symptoms while running in the ON position, why does the engine start again when the lever is turned to RES? The reason can be seen by looking at the two pipes that are inserted into the tank. The two pipes lead to the ON and OFF passages in the cock, and by changing the length of the pipes, the position of the gasoline in the tank is changed.

While there is a large amount of gasoline remaining in the tank, it is sucked in through the long pipe leading to the ON, and when the tip of the long pipe sticks out of the gasoline in the tank, it is switched to the RES leading to the short pipe, which allows the gasoline to continue flowing close to the bottom of the tank.

The difference in the length of the two pipes increases or decreases the distance that can be traveled in the RES position. If there is a large difference in pipe length between ON and RES, the shorter the time that can be traveled in the ON position, the longer the time = distance that can be traveled in the RES position. Conversely, if the pipe length difference is small for the same tank, the distance that can be traveled in the ON position will increase, but the time that can be traveled in the RES position will decrease.

The difference in pipe length is determined by the motorcycle manufacturer based on the fuel consumption of the model, etc. However, the pipe press-fitted into the cock can crack or come loose due to age-related deterioration or trouble. For example, if the pipe on the ON side comes loose from the base, the lever will continue to suck gasoline to the bottom of the tank while it is ON, and the tank may be empty when the lever is switched to RES.

Dirt and rust in the fuel tank can also clog the strainer on the RES side pipe near the bottom, reducing gasoline flow and causing the tank to stall.

In addition to the pipes that are inserted into the fuel tank, there are also defects in the packing of the switching lever that allows fuel to leak out of the cock. The lever selects one of the two pipes, ON and RES, for gasoline to flow to the carburetor side, and the rubber packing ensures the airtightness of the flow path.

However, if the packing wears out due to repeated operation of the lever, or if it deteriorates or cracks due to long-term storage or neglect, gasoline will leak. In this case, it is necessary to replace the packing at the base of the lever.

If the plate that holds the lever is screw-fastened and the packing behind the lever is set as a replacement part, the repair is possible, but some models have a non-disassembled lever part and the packing is not set as a single item and can only be replaced with a fuel cock assembly.

Damage to the diaphragm of the negative pressure cock will cause gasoline to stop flowing.

When the diaphragm cover on the back side of the cock is removed, a spring is revealed that pushes against the valve that closes the passageway. When enough negative pressure is applied to this negative pressure chamber to overcome the tension of the spring, the valve on the back side of the diaphragm opens and gasoline flows from the fuel tank to the carburetor. When removing the diaphragm cover to replace the O-ring attached to the end of the valve, which is sold as a part, carefully pull off the diaphragm stuck to the cover to prevent damage.

The inside of the fuel cock attached to a rusty fuel tank is often clogged with dirt and rust, so so soak it in carburetor cleaner and clean it. Dirt on the pipe's strainer surface should also be cleaned with a toothbrush. Some discontinued or old motorcycle cocks whose pipes are made of brass may have the strainer at the top fall off. In such cases, a fine brass net can be formed into a bag shape with solder and soldered to the pipe.

When overhauling the fuel cock, replace not only the rubber packing behind the lever but also the O-ring on the outer circumference of the lever with a new one. Degreasing the cock body with parts cleaner will reduce the sliding of the O-ring, so it is recommended to apply a thin layer of silicon grease when assembling the cock body.

Along with the packing at the base of the lever, the diaphragm is the key component of the negative pressure cock.
As mentioned earlier, even if the lever is set to the ON or RES position, gasoline cannot flow through the negative-pressure cock unless a passage is opened by the diaphragm, a thin film of rubber inside the cock, due to the negative pressure generated by the engine.

Therefore, if the diaphragm is punctured or torn for some reason, or if the O-ring on the valve that is opened and closed by the diaphragm sticks, gasoline will not be supplied to the carburetor even if negative pressure is applied to the fuel cock.

If the fuel tank has enough remaining but the float chamber is empty and the motorcycle stalls, the condition of the diaphragm must be checked as well as the clogging of the two pipes of the cock.

In the case of the fuel cock for the Kawasaki Zephyr shown here, the circular packing at the base of the lever is sold as a part, while the diaphragm is not available as a part and must be purchased as a whole cock. However, the small O-ring for the valve that opens and closes the gasoline passage at the center of the diaphragm is supplied as a single item.

In addition to ON and RES, some negative pressure cocks have a position called PRI. The role of PRI, which stands for PRIMARY, corresponds to the ON position of a gravity cock, and features gasoline flow even when no negative pressure is applied.

Therefore, if the engine runs out of gas in the ON or RES position, while it starts normally in the PRI position, the diaphragm part, which should operate under negative pressure, is considered defective. On the other hand, if gasoline does not flow to the carburetor in PRI, ON, or OFF position even though there is gasoline in the fuel tank, the two pipes of the cock are probably clogged.

A typical pattern of fuel cock problems is that gasoline seeps from the lever or does not flow to the carburetor because the packing rubber hardens after a long period of non-riding for various reasons, or because of the gasoline inside the cock changes and clogs. If the cock goes bad, understand the features of the negative pressure cock and perform appropriate maintenance.