【PR】Kishida Seimitsu Kogyo Company KEYSTER
Key Ster’s fuel adjustment kit, which enables the setting of the OEM carburetor by combining the pilot jet, main jet, and jet needle, is relied upon by many out-of-print car users.
However, depending on the original condition of the carburetor, some carburetors cannot be completely restored with a fuel adjustment kit alone. For such severely damaged carburetors, Key Ster has developed a product that is different from existing fuel adjustment kits. That is the Valve Seat Revival Kit and Overflow Pipe Set for Honda CB750F. Here we introduce two new products that can reliably repair essential parts.
- What is the importance of needle valves and valve seats?
- There are two types of valve seats: removable and non-removable.
- Valve seat revival kit developed to replace non-removable valve seats
- What is the role of the overflow pipe press-fitted into the float chamber?
- If the overflow pipe is damaged due to aging or accidental damage during maintenance
- Overflow pipe set to replace damaged overflow pipes
What is the importance of needle valves and valve seats?
The negative pressure generated by the engine causes the fuel in the float chamber to be sucked up into the venturi to create the mixture in the carburetor. The fuel, which is metered by the pilot jet, main jet, and jet needle depending on the magnitude of the negative pressure and the intake airflow, is all supplied from the float chamber. Fuel flowing from the fuel tank to the carburetor passes through the valve seat and accumulates in the float chamber, causing the float to float. When the specified amount of fuel has accumulated, the tip of the needle valve at the base of the float adheres to the valve seat and stops the inflow of fuel. The fuel in the float chamber repeatedly opens and closes the needle valve, which moves in conjunction with the float, to control the height of the fuel level in the float chamber, which is the most important key point in the carburetor setup.
The needle valve opens when the fuel in the float is reduced and closes when the oil level rises. This is a simple operation, but repeated over decades can cause wear and tear on the contact surfaces of the valve and valve seat. Even if the oil level in the float chamber rises and the float closes the needle valve, fuel continues to flow in through the small gap between the valve seat and the oil level, causing an overflow. Although neither the needle valve nor the valve seat is a component that affects the setting of the carburetor like the jet or the needle, they are in fact the most important and fundamental parts of the carburetor.
The valve seat (ochre-colored circular part on the carburetor side) of the OEM carburetor for the Honda CB750 F is press-fitted at the time of manufacture. It can be pulled out by pulling it up vertically, but it cannot be replaced because there are no OEM parts available. In reality, however, there are cases where the part that the needle valve contacts are worn or damaged and fuel leakage does not stop even if only the needle valve is replaced.
There are two types of valve seats: removable and non-removable.
The needle valve, which controls the flow of fuel by contacting the valve seat, is also included in Key Star’s fuel adjustment kits and can be replaced with a new one if the conical contact area is worn or damaged.
A common case of needle valve trouble is fuel leakage with traces where it contacts the valve seat.
If the needle valve is normal, the contact area with the valve seat is linear contact.
However, when the contact area wears out due to repeated opening and closing, the contact width becomes wider.
This is what is called a sticky contact, and the force exerted by the float is dispersed, which reduces the pressure to close the valve and causes fuel to flow into the valve.
When the needle valve hit area wears out, the contact surface of the mating valve seat is also likely to be worn or damaged. Therefore, if the valve seat can be removed from the carburetor body, it is best to replace the needle valve and valve seat as a set. Fuel adjustment kits for models with OEM carburetors that allow the needle valve to be removed include a set of replacement needle valves. The problem here is the type of carburetor in which the valve seat is press-fitted into the body and cannot be removed. If the contact surface of the valve seat is worn and damaged, even if the needle valve is replaced with a new one, the valve may not close completely and overflow may occur. If the surface is only lightly soiled or rough, the valve seat contact surface can be prepared with a cotton swab moistened with metal polishing chemical, but if it is damaged or corroded, it may not be repairable. Even if a damaged valve seat could be removed, the conventional wisdom was that the damaged valve seat could not be repaired by replacement because there were no OEM parts available for valve seats that were not designed to be removed, i.e., fatal damage.
Valve seat revival kit developed to replace non-removable valve seats
Even with the vast lineup of Key Star fuel adjustment kits, the problems associated with carburetors that do not have removable valve seats were similar.
Even if the needle valve is new if the valve seat is in poor condition, the oil level in the float chamber will not be stable, and as a result, carburetor settings will not be determined.
The new Valve Seat Revival Kit is a solution to this situation. The kit consists of a valve seat of the same shape and size as the factory valve seat, a needle valve, and a special tool for pulling out the factory valve seat and installing a new valve seat. The new product is a valve seat for the OEM Keihin VB series carburetor for the Honda CB750F. The valve seat diameter (the part opened and closed by the needle valve), which meters the fuel flowing into the float chamber, as well as the outer and inner diameters and overall length, are the same size as the CB750F.
For users with some experience and knowledge of carburetors who have had the frustration of reusing press-fit valve seats that could have been reused if only a replacement needle was available, the Valve Seat Revival Kit is the perfect product for them. Although it may seem difficult to remove and install a press-fit valve seat, the kit features a special tool that makes the job easier than you might imagine. In the case of the CB750F carburetor, there is no adjusting plate for oil level adjustment on the float body, and the oil level is determined by the amount of valve seat press-fit, so careful work is required during assembly, but the great news is that the valve seat that has deteriorated due to wear and tear can be replaced with a new one. The press-fit type valve seat is a method often used not only for the CB750F but also for other Keihin carburetors, and we plan to develop a valve seat revival kit for the CVK, which is synonymous with negative pressure carburetors.
The CB750F is a popular model, but the CVK carburetor can be used in a wide range of models, from 250cc class to big bikes over 1000cc, so we believe that this product will be useful for many more users.
Two types of valve seat revival kits are available for the Honda CB750FZ (for carburetor No. VB52BA) and CB750FB/FC (for carburetor No. VB52BB/VB52EA/VB52EB). The kit includes 4 valve seats and 4 needle valves each, and a jig for pulling and press fitting. The price is (approx.) 67 USD per kit including tax.
To remove the old valve seat, cut the female threads with the provided tap. The tap handle is not included, so the operator must provide it. Cut the threads until the tip of the tap touches the bottom of the valve seat.
The cylindrical jig is placed over the valve seat, and a long bolt threaded through the nut provided is screwed into the previously female-threaded valve seat. Turning the nut while holding the head of the bolt in place gradually pulls up the valve seat, which is press-fitted into the carburetor body.
On the right is a new part from the valve seat revival kit, and on the left is an old valve seat pulled from the body. The valve seat made by Key Star matches the factory size not only in appearance but also in the diameter of the fuel passage, which is important for the valve seat.
Set the press-fit frame in the cylindrical jig and press-fit the valve seat. Instead of slamming it in from the beginning, fine-tune the valve seat with your finger so that it does not tilt before placing the jig over it.
Rather than hitting it with a hammer, the weight of the head is used to gradually press-fit it.
In the case of the CB750F carburetor, there is no adjusting plate for oil level adjustment on the float, so the height of the float is determined by the amount of press-fitting of the valve seat. If the amount of press-in is too shallow, the float will swing down and the oil level will be low. If the amount of press-in is too deep, the float will swing up and the oil level will be high. If the valve seat is pressed in too deeply, it cannot be pulled out, so the float is temporarily attached with a shallow press-fit and measured with a float gauge, and when the gauge oil level is confirmed to be low, it is pressed in a little deeper and measured again to set the standard value of 15 mm.
Replacing the valve seat and needle valve as a set stabilizes the oil level, which is most important for carburetors. If the oil level rises when fuel is actually flowing, even though the static oil level is adjusted with a float level gauge, the valve seat should be replaced as well as the needle valve. Until now, this was not possible with the press-fit type, but with the introduction of the Valve Seat Resuscitation Kit, more ideal maintenance is now possible.
What is the role of the overflow pipe press-fitted into the float chamber?
Although not related to carburetor setup, the overflow pipe is one of the most important parts of the carburetor along with the valve seat. The overflow pipe, which is press-fitted into the bottom of the float chamber, drains excess fuel out of the carburetor in the event that the oil level in the float chamber rises excessively due to trouble with the float, needle valve, or valve seat.
If the oil level in the float chamber continues to rise without an overflow mechanism, there is a risk of fuel entering the venturi and flowing into the engine or air cleaner box. Some carburetors, such as the Keihin CVK, are not equipped with an overflow pipe from the design stage, but it plays a more important role than one might imagine.
An overflow pipe press fitted into the bottom of the float chamber extends somewhat above the surface where it meets the body. When the fuel oil level in the float exceeds the top of the pipe, it is discharged out of the pipe, preventing the mixture from becoming excessively thick. Not all carburetors are equipped with an overflow pipe, and some carburetors do not even have one.
If the overflow pipe is damaged due to aging or accidental damage during maintenance
The overflow pipe itself is a hollow metal pipe, but it can develop holes or cracks in its sides due to aging or corrosion, and when this happens, fuel can leak out at a stage where the oil level in the float chamber is lower than normal, causing the carburetor setting to become thin.
Also, in the case of out-of-print cars, the overflow pipe may hit somewhere and break while attaching and detaching the float chamber many times. In this case, as with cracks and holes, fuel will flow out of the carburetor when the oil level reaches the top of the pipe. The worst case is when the pipe breaks from the base, and in this case, the fuel in the chamber is reduced to almost zero, making it impossible to start the engine or run the car.
Similar to press-fit type valve seats, overflow pipes that are not intended to be replaced are not available as OEM parts. If a crack appears on the side of the pipe, it can be sealed by pouring solder. However, if it is broken from the root, it cannot be dealt with. If OEM parts are available, a new replacement can be made, but for out-of-print cars, for which parts are often no longer sold, the common method of dealing with the problem was to find used parts that can be used for the overflow pipe and replace it.
These carburetor overflow pipes are also corroded by long-term neglect, but some deteriorate so that they develop holes or longitudinal cracks. If a hole develops in the middle of the pipe, the oil level can only go up to that level. Depending on the location of the hole or crack, if the oil level leaks at a low level, the float will not rise and the needle valve will not close, allowing fuel to continue flowing. If the fuel cock is a negative pressure type, or if it is a gravity cock and remains on, it will continue to leak.
Overflow pipe set to replace damaged overflow pipes
Key Ster’s new overflow pipe set is a replacement part for cracked or broken stock overflow pipes. Currently commercialized are three types: for Honda CB750F series (FZ to FC) Keihin VB 4-cylinder vehicles, for Honda Hawk II and GL series Keihin VB 2-cylinder vehicles, and for Kawasaki W1S to W3. All of them have the same pipe outer diameter and overall length as the factory unit. The old overflow pipe is removed and press-fitted using the hammering jig provided with the set.
The process is simple, but the difficulty of the work depends greatly on whether the old overflow pipe can be pulled out smoothly. If the pipe is a float chamber that has become less press-fit due to aging, it may be relatively easy to pull it out with pliers or pliers. However, if a pipe that cannot be easily pulled out is grabbed with pliers, it may be severed in the process. Also, a pipe that is broken from the base cannot be grabbed with pliers.
In this case, a drilling machine can be used to remove the remaining broken pipe; in the case of the CB750F, the outer diameter of the pipe is 3 mm, so a 3 mm drill can be used to drill a hole vertically to remove the tip of the remaining pipe.
It is not impossible to work with a hand-held drill, but there is a risk that the blade will tilt and enlarge the hole in the press-fit area, causing the pipe to govern, so it is better to utilize a drilling machine to reduce the risk. It is not difficult to press-fit a Key Star overflow pipe manufactured to the same dimensions as the factory pipe, although care must be taken when pulling out the old pipe. The brass pipe is soft and delicate, but the included hammering jig can be used to press-fit the pipe without crushing the end or bending it in the middle. If you press-fit the pipe until the tip of the jig touches the float chamber, the amount of pipe protrusion will be the same as the OEM pipe, so there is no need for fine-tuning.
If you know that the height of the oil level in the float, which is the standard for carburetor setup, changes depending on the condition of the overflow pipe, you will understand the effectiveness of the overflow pipe set.
Overflow pipe set consisting of an OEM size overflow pipe and hammering jig. Keihin VB carburetors for Honda CB750FZ to FC (carburetors No. VB52BB/VB52EA/B52EB) are 33 USD per set including tax; for Honda Hawk II series and GL series Keihin VB 2-cylinder cars and Kawasaki W1S to W3, 2,750 yen per set including tax.
If the press-fit portion of the pipe is thin due to corrosion or other reasons, it may be possible to pull it out with a pair of pliers. On the other hand, if the press-fit is in good condition and strong, the pipe may be crushed and cut if the pliers are grabbed tightly.
If excessive force is applied to the pipe when attaching or detaching the float chamber, it may break off from the press-fit part. In such cases, a drill is used to rub out the remaining pipe in the float chamber.
The outside diameter of the overflow pipe was ø3 mm, so we used a 3 mm drill to drill through the pipe, and in the process the tip of the pipe became entangled with the drill blade and we were able to pull it out. With a hand drill, there is a risk that the blade will tilt and damage the float chamber body, so we would like to work with a drilling machine.
If the float chamber side is intact, fuel will not leak from the press-fit area, but just to be safe, apply a fuel-resistant type liquid gasket to the end of the pipe before press-fitting.
Place the hammering jig over the pipe and lightly tap it with a hammer to press it in. Press-fit the jig until the tip of the jig contacts the float chamber.
Float chamber with brand new overflow pipe shining. It is good news for users of discontinued vehicles that float chambers with holes or broken pipes can be reused.