The fuel cock used to switch the flow path between the fuel tank and the carburetor generally incorporates a rubber packing that ensures both operabilities of the switching lever and fuel tightness. If this packing deteriorates, it can cause fuel leakage, but depending on the construction of the cock, it may be possible to repair it by replacing internal parts.
Rubber gaskets that deteriorate when left after fuel comes in contact with them
The fuel cock of the Chappie, Yamaha's leisure bike introduced in 1973, was a negative pressure type from the initial model. When targeting female users who were not familiar with motorcycles, Yamaha may have wanted to save them the trouble of turning the cock off every time they stopped the engine. However, in addition to ON/RES, a PRI position is provided so that fuel can flow to the carburetor without starting the engine when necessary.
The fuel cock is installed on most carbureted vehicles from mopeds to large cars except scooters. The fuel cock, which switches between ON and RES reserve depending on the amount of fuel remaining in the tank, is installed between the fuel tank and the carburetor and is classified into gravity type and negative pressure type depending on its structure.
The gravity type has an OFF position in addition to ON and RES. If the lever is in the OFF position, fuel is shut off from the cock to the carburetor. On the flip side, when the lever position is ON or RES, fuel always tries to flow through the cock and into the carburetor. If there is fuel in the float chamber of the carburetor, the float valve stops the flow and fuel also stops. However, if a foreign object gets caught between the float valve and valve seat, or there is a problem on the carburetor side such as a punctured float that reduces buoyancy, fuel If a foreign object gets caught between the float valve and the valve seat, or if the float is punctured and its buoyancy is reduced, fuel will continue to flow into the float chamber, causing overflow, so the theory is to keep the lever OFF when the motorcycle is parked or in storage.
In contrast, the negative pressure cock interrupts the fuel flow by activating the diaphragm of the cock with negative pressure extracted from the intake side of the engine. The diaphragm, which is a thin film of rubber, is subjected to a force exerted by the spring in a direction that stops the flow of fuel so that no fuel flows into the carburetor when the engine is stopped. When the engine is started, the negative pressure applied to the diaphragm causes the spring to contract, opening the flow path and allowing fuel to flow into the carburetor through the ON or RES selected by the lever. Therefore, when the motorcycle is stored, the carburetor will not overflow on its own, even if the lever position is ON or RES.
However, the situation is slightly different for a negative-pressure cock with a PRI primary position. The PRI acts as a bypass for fuel to flow even without negative engine pressure so that if, for example, the carburetor is overhauled and the float chamber is emptied, fuel can be supplied even if the engine is not started. However, it should be noted that if the motorcycle is stopped with the PRI set, it will be in the same state as when the gravity cock is turned on, and fuel will always flow from the fuel tank toward the carburetor.
A rubber packing is incorporated in the mounting portion of the lever that switches the fuel flow path, and its elasticity ensures smooth lever operation and prevents fuel leakage from the flow path. Cock packings come in a variety of shapes, but the most common is a disk-shaped packing with several holes on its surface. Two of these holes are connected to the ON and RES fuel outlets on the fuel tank side, while the other leads to the carburetor. If the negative pressure cock has a PRI, there may be a fourth hole that serves as a diaphragm bypass flow path.
These gaskets are made of rubber that is resistant to fuel, but deterioration is inevitable over time. If used frequently, this is not a problem, but if the packing dries out after a long period of inactivity, it may lose its flexibility or crack. In the case of negative-pressure cocks, as mentioned earlier, when the diaphragm closes the flow channel, fuel does not come in contact with the cock packing, which is not the case for some cocks depending on the internal structure. If the fuel in the cock lever area volatilizes after a long period of storage, the packing will dry and begin to deteriorate.
And as this packing deteriorates, fuel seeps out of the lever. In the case of the models shown here, fuel flows to the lever when the diaphragm opens due to negative pressure, so when the engine is started, fuel seeps out slowly at the base of the lever. PRI also seeps in the same way.
POINT
Point - A rubber gasket is incorporated at the base of the fuel cock lever to prevent fuel leakage while allowing the lever to operate smoothly.
Point 2 - Deterioration over time, among other things, once fuel comes in contact with the packing, drying can cause the packing to harden and fuel leakage to occur.
If the packing is a replaceable cock, assembly replacement can be avoided.
After more than 40 years without replacement, the lever gaskets had become as hard as plastic and had numerous fine cracks. There is nothing that can be done if genuine parts are not available or if it is a non-disassembled type, but greasing the ticking gasket may temporarily restore functionality, but fortunately, genuine parts are still available today, so it will be replaced with a new one.
New gaskets were also obtained for the cup-filter mating area at the bottom of the cock. The old packing is not flexible and does not respond at all to being collapsed when the cup is tightly tightened. If the cup is over-tightened to prevent fuel leakage with this packing, the soft zinc die-cast material may be damaged, so it should be replaced together with the base of the lever.
When this happens, the fuel cock, which is a non-disassembled type and is only available as an assembly, has to be replaced in its entirety. If the vehicle becomes out of print and the parts are no longer sold, it will be a considerable blow to the owner.
On the other hand, some cocks can be disassembled and internal parts are supplied individually. The cock pictured here is from a model manufactured 48 years ago, but the genuine packing for the lever and cup filter is still supplied. Since these parts are often common across models, it is likely that they have been used in later models and are therefore still available.
To give an example of a newer bike, the fuel cock of the Honda CB400 Super Fore in the carburetor era was non-disassembled and supplied only as an assembly, while the same 400cc naked Kawasaki Zephyr had a disc-shaped packing and O-ring at the base of the lever that could be purchased separately, disassembled, and However, the same 400cc naked bike, the Kawasaki Zephyr, had a disc-shaped packing and O-ring at the base of the lever that could be purchased separately. However, in the case of some popular models, aftermarket parts suppliers may have released compatible fuel cocks, so there are cases where repairs can be made with non-original parts.
The most popular fuel tank-related problem is rust on the tank itself, but the second most frequent problem is seepage or leakage from the fuel cock. If your car's fuel cock can be disassembled and gaskets are available, we recommend replacing it while symptoms are mild.
POINT
Point 1 - There are two types of fuel cocks: the non-disassembled type, which is only available in assembled condition, and the type whose internal parts can be replaced.
Point 2 - The fuel cock with replacement gaskets available can be repaired even if the fuel cock is older.
