Compatible with 500 Different Models! KEYSTER Carburetor Fuel Tuning Kit Meets All Needs from Overhauling to Tuning

000bbb-min-1.jpg How to

The carburetor, which supplies fuel to the engine in response to the air drawn in by the engine, is a sophisticated and precise key component that makes clever use of the natural order of things. All new motorcycles nowadays are equipped with fuel injection. However, a large number of motorcycles sold in the past and present in the market are equipped with carburetors, and thus require proper maintenance and tuning to recover from aging and replacement of engine-related parts.

The "Carburetor Fuel Tuning Kit" developed, manufactured and sold by Kishida Seimitsu Kogyo, which is celebrating its 70th anniversary, is a part that enables the setting of the OEM carburetor when installed on a new motorcycle. In this article, I would like to review the structure and principles of carburetors, as well as discuss the features and applications of Carburetor Fuel Tuning Kit.


002ab-min.jpgIn order to disassemble the OEM carburetor and get replacement parts, each part has to be ordered separately. In addition, parts such as jets and needles are only available in one size, which is the OEM specification for most models. The "Carburetor Fuel Tuning Kit" manufactured by Kishida Seimitsu Kogyo, known under the brand name "KEYSTER," is a revolutionary product that overturns such common sense. The price of the kit "FY-5156N" for Yamaha SR400 (2001-2008 models) is 36.35 USD including tax.
003a.bjpg-min.jpgThe main feature of the Carburetor Fuel Tuning Kit is that it provides main jets, pilot jets, and jet needles of different sizes, mainly standard ones, although the contents of the parts may differ depending on the model. Even if the engine is OEM, changing the exhaust system may affect the intake air along with the change in exhaust efficiency, and this is where the advantage of the fuel adjustment kit comes into play, as it allows you to change the settings with the OEM carburetor.


The pilot jet, jet needle, and main jet are three sets that come in handy when setting up the carburetor. With the exception of a very few models, the motorcycle manufacturer's OEM parts have only one type of jet and needle, leaving no room for setting.

Carburetor settings can be changed simply by replacing the air cleaner box and exhaust system.

Air, fuel, and sparks are essential to keep the engine performing well. The carburetor is an important component that mixes air and fuel to produce a mixture. In the modern age of fuel injection, some riders may think that carburetors are prehistoric and inferior in performance, but this is not the case.

The carburetor is a very precise metering device that is also exquisitely fuzzy. The carburetor is designed so that the right amount of fuel is always sucked out of the float chamber according to the amount of negative pressure and air flow that varies according to throttle opening, engine speed, and load. Compared to fuel injection, which can only inject fuel according to a pre-set program, it has the advantage of operating based on the natural order of things.

The carburetor contains various components such as main jets, pilot jets, and jet needles in order to produce the proper air mixture from idling to high RPM. These are not just engine specifications, but also the combination = setting is made to work best in a total package including the air cleaner box to the exhaust end.

Therefore, if you install an exhaust system that has less exhaust resistance than the OEM part, more air will flow into the engine even under the same driving conditions, and the mixture will become thinner unless you use more fuel. The same thing happens when the air cleaner box is replaced with a power filter. The air cleaner box reduces the resistance and the negative inlet pressure, so you need to increase the fuel to keep the balance.

People tend to think that setting up the carburetor will increase the engine's power, but in reality, the original purpose of the carburetor is to rebalance the intake and exhaust air, which is changed by replacing some parts. It is important to understand that the purpose of the carburetor setting is to adjust the fuel to match the amount of air that is now entering the engine compared to the OEM condition.

005bb-min.jpgThe 'Skeleton Tuning', which removes the air cleaner box, is a STD style for single models. By removing the air intake box, the carburetor will be able to breathe more air, and this is often thought to lead to tuning, but since the negative pressure on the carburetor changes significantly, setting is essential. To begin with, universal racing carburetors, which are designed to be set up individually, have a wide variety of jets and needles in different sizes, but the OEM carburetors are set to specifications by the motorcycle manufacturer and cannot be adjusted.

006bb-min.jpgA well-draining exhaust system improves exhaust efficiency, which leads to better engine performance, but it also means that intake air must be pumped in to balance the exhaust flow. When more air is taken in, the fuel has to match it. This is the reason why when an exhaust system is replaced, not only the exhaust system but also the carburetor setting is required at the same time.

Carburetor Fuel Tuning Kit enables setting of OEM carburetor.

One of the reasons why racing carburetors have attracted so much attention in carburetor tuning is because of the wide variety of setting parts available. To begin with, racing carburetors are universal parts and need to be able to adapt to the engine specifications of the model being installed, which is why the jets and needles are set in detail.

In contrast, the OEM carburetor was developed by the motorcycle manufacturer to give the best performance with the OEM intake system and exhaust system, and since it is not designed to be modified, there are no setting parts. Therefore, even if the balance of the intake/exhaust system was changed by replacing the air cleaner box or exhaust system, it was not possible to take effective measures.

KEYSTER Carburetor Fuel Tuning Kit is a revolutionary product that allows you to set the OEM carburetor where there was no way to set it before.

Carburetor Fuel Tuning Kit is available for about 500 models from 50cc to over 1000cc from four Japanese manufacturers from the 1960s to the 2000s. The main jet, pilot jet (slow jet), and jet needle, which are the key components for setting up the jet, are mainly OEM and are available in several sizes, although the detailed assembly differs depending on the model.

The setting factors are differentiated according to the throttle opening: pilot jet = low opening, jet needle = medium opening, and main jet = high opening. However, each of these functions does not switch digitally, but rather overlaps with each other, for example, fuel is sucked out of the pilot jet even at medium opening. The theory behind the actual carburetor setting is that the pilot jet, jet needle, and main jet should be set in that order, and the jet needle setting is essential for smooth rotation from the pilot area.
The same goes for the connection from the jet needle to the main jet.

Some parts manufacturers sell jets of various sizes individually, but KEYSTER has developed a set of three related parts: pilot, main and needle.

While the STD size jets and needles follow the motorcycle manufacturer's OEM size, other sizes are KEYSTER's own size settings. This is because the mixture state = air-fuel ratio is developed to change in stages when the pilot jet and jet needle or the jet needle and main jet are combined. By ensuring that changes to the jets and needles are reflected in the air mixture, the user can easily feel the changes in the settings.

007cb-min.jpgThe Yamaha SR400 attracted many motorcycle fans for a long period of time from 1978 to 2021. Since 2010, it has been equipped with fuel injection, but between 1978 and 2008, four different types of carburetors were used. From 2001 to 2008, it will be equipped with a negative pressure type Mikuni BSR.

008cb-min.jpgRemove the carburetor from the motorcycle, drain the fuel from the float chamber, and remove the four screws to open the float chamber. The carburetor body is lined with narrow passageways for air and fuel to pass through, and if fuel that has deteriorated or rusted due to long-term storage or neglect gets stuck or clogged, it needs to be cleaned before setting because it changes the flow of air and fuel.


The starter jet is a jet that functions only during startup, and there are two jets in the float chamber that are changed during carb setting: the pilot jet and the main jet.

010cb-min.jpgWhen attaching and detaching the pilot jet and main jet, use a flathead screwdriver that exactly matches the width of the groove, or a special carburetor jet driver. The jet is made of a soft brass material, so be careful if there is a difference between the width of the groove and the thickness of the screwdriver, as it is easy to strip. When loosening the main jet, it tends to turn together with the main nozzle holder at the bottom, so it is recommended to use a wrench to stop it from turning.

011cb-min.jpgThe main jet, which affects the medium to high-speed range, is available in six different sizes from #157.5 to #175, with the manufacturer's OEM #162.5 as the STD. If the amount of air that the engine draws in is increased, such as by increasing the bore, the size is increased in order to increase the amount of fuel. The pilot jets, which affect the idle to low-speed range, are three sizes, #21 to #29, with the OEM #22.5 being the most common. There is no setting factor for the starter jet, and the same #60 as the OEM jet is available as a replacement.

012cb-min.jpgA needle valve with a black conical tip attached to the float opens and closes in the valve seat on the carburetor body side to interrupt fuel flow from the fuel tank. It is important that the needle valve and valve seat open and close without friction loss and that the valve does not slant.


The needle valve can be removed from any carburetor, but some carburetors have valve seats that are press-fitted and cannot be removed. The needle valve in KEYSTER's fuel tuning kit is made of alcohol-resistant rubber that does not deteriorate with biofuel.

014cb-min.jpgWhen replacing the jet needle, pull out the negative pressure piston from the top of the body. Remove the piston cover, pull out the long spring, and take out the piston, making sure that the thin rubber diaphragm is not cracked or damaged. If the diaphragm is damaged, even if the negative pressure in the cylinder increases, the negative pressure piston will not be able to rise and enough air will not be taken in, so the engine speed will not increase.

015cb-min.jpgThe configuration of the parts varies depending on the type of carburetor, but the jet needle inserted into the bottom of the negative pressure piston is not completely fixed, but has a spring built in to allow it to move up and down, left and right, within a small range. The slight movement of the jet needle centers it in the middle of the main nozzle even when the negative pressure or the size of the inlet air changes.

016cb-min.jpgThere are four sizes of jet needles, mainly the OEM size. The fuel that is metered by the main jet and rises up the main nozzle is sucked out into the carburetor passage through the gap between the needle and the needle stuck in the main nozzle. Jet needles have two elements: the diameter of the straight portion and the taper angle, which tapers toward the tip. The narrower the straight diameter and the stronger the taper angle, the more fuel is released, resulting in a thicker mixture ratio. Also, when the clip position is raised, it becomes lighter, and when it is lowered, it becomes darker.

017cb-min.jpgThe pilot screw is important for adjustment at idling when the throttle valve is closed, and the precision of the fine needle at the tip is critical. There are two types of idling adjustments: the pilot screw type, which adjusts the amount of air mixture after air and fuel are mixed, and the air screw type, which adjusts the amount of air entering the carburetor. In the case of the pilot screw that adjusts the amount of mixed air, an O-ring for sealing is built into the tip of the screw because the mixture ratio will change if outside air is sucked in through the base of the screw. When attaching and detaching the screw, be careful not to forget to attach the O-ring or leave it in the carburetor body.

The convenience of providing even parts that are not available as single items from the factory is also attractive.

The jets, needles, float valves and valve seats, and float chamber gaskets included in KEYSTER's Carburetor Fuel Tuning Kits for about 500 models, both old and new, and of any displacement, are developed and manufactured by the company itself. As a result, they have different features and personalities from the OEM parts of motorcycle manufacturers.

The needle valve (float valve) and valve seat, which control the flow of fuel into and out of the float chamber, are parts that determine the oil level, one of the most important factors for carburetors. It is also required to be compatible with biofuels including alcohol. Biofuels were developed to address environmental issues, but it has been pointed out that prolonged immersion of the needle valve in the float chamber can cause malfunction.

To solve this problem, KEYSTER has developed and patented the AA (Anti-Alcohol) Needle, which is made of a rubber material that provides high durability against alcohol in fuel. KEYSTER's initiative is welcome news for carburetor motorcycle users, as it is unlikely that motorcycle manufacturers who have shifted from carburetors to injection will now develop updated parts for carburetors.

In addition to jets and needles for setting, the Carburetor Fuel Tuning Kit features a full range of essential parts for overhaul and maintenance. The kit for Yamaha SR400 (2001-2008 models) shown here includes an O-ring for the starter plunger. As a general rule, the rubber O-rings should be replaced with new O-rings once they are removed, as they crush and provide an airtight seal. However, the O-ring is not available as a single item in the OEM parts, and can only be obtained as a set with the plunger. The same is true for the pilot screw O-ring, which is used to adjust the delicate mixture in the idling range, and although it is not available separately, both are included in the fuel adjustment kit.
Also, it's for a different model than SR but an air cut valve (coasting enricher), which is important for controlling the air flow when the throttle is fully closed, has also been developed and commercialized independently.

018db-min.jpgA starter plunger that sends thicker fuel when cold to improve starting. Some models use a choke during cold start, but in the case of a choke, the air passage is closed so that the negative pressure on the engine side works strongly to thicken the mixture. The purpose is the same, but the starter plunger allows more fuel to flow, while the choke increases the negative pressure by blocking the air.

019db-min.jpgThe O-ring that is installed on the threaded part of the starter plunger of SR400 is not sold separately, and if it is damaged during installation or removal, the plunger must be purchased as a set. The suggested retail price for the set of parts is 29 USD including tax (*as of August 2021), which is rather expensive if you think it is for one O-ring. The fuel adjustment kit also has an O-ring for this part, which can be replaced during maintenance or overhaul.

The Carburetor Fuel Tuning Kit is useful not only for tuning, but also for maintenance and restoration.

KEYSTER's Carburetor Fuel Tuning Kit is a part for setting and tuning, but as mentioned above, it is also very valuable as an overhaul kit because of its wide range of O-rings and gaskets.

With the recent popularity of discontinued models and old motorcycles, carburetor maintenance is essential. In the past, it was common to give up on OEM carburetor that was in bad shape and replace it with a racing carburetor, but there are many fans of discontinued models who respect the STD style. However, the OEM parts of motorcycle manufacturers are set up separately for each individual part, and sometimes you only need an O-ring, but it is only available as an assembly. Furthermore, without a parts list at hand, it is difficult to pick up the parts you need.

In the case of Carburetor Fuel Tuning Kits, the settings are model and year specific, so you don't need to know the OEM numbers of the main jet and needle valve to get the parts. It is an excellent maintenance item that even includes parts that should be replaced during disassembly, such as the top cover gasket, float chamber gasket and O-rings.

Some may argue that if everything from the air cleaner box to the exhaust system is OEM, the OEM size jets and needles are sufficient. However, as the years go by since the motorcycle was new, the condition of the engine itself may deteriorate, or on the other hand, the performance of the engine itself may be improved from the STD condition by oversized pistons or cylinder head modification.

To cope with such changes, a fuel tuning kit allows you to reduce or increase the amount of fuel. In addition to the OEM specifications of a new motorcycle, the most important role of tuning is to make the best settings for the current situation. The Carburetor Fuel Tuning Kit is not necessarily a product for customization or modification only, such as increasing the bore or changing the exhaust system.

What is noteworthy is the reasonable price tag. The basic price of KEYSTER's Carburetor Fuel Tuning Kit is 36.35 USD including tax per cylinder of carburetor (may vary depending on the contents of the set). This is surprisingly inexpensive compared to motorcycle manufacturers' OEM parts prices. The total price of OEM parts for Yamaha SR400 such as main jet, pilot jet, jet needle, float valve seat, pilot screw, float chamber gasket, etc. is over 135 USD including tax, and the difference is obvious. Of course, there is no choice in jet or needle size for OEM parts.

With a single cylinder engine, the difference is so great that it becomes even greater for a four-cylinder engine with a quadruple carburetor, and the superiority of the Carburetor Fuel Tuning Kit becomes obvious.

From carburetors to fuel injection, future cars and motorcycles are trying to shift away from fuel engines themselves. While such a trend is unavoidable when considering the global environment, if you want to continue your motorcycle hobby at the user level, getting in touch with a carburetor should be a great pleasure.

From the most popular discontinued models to the rarest of rare models, KEYSTER's Carburetor Fuel Tuning Kits are available in a wide variety of product lines. This item is recommended for all carburetor owners who want to practice tuning, restoration, and tinkering with their motorcycles more deeply.

020eb-min.jpgThe O-ring type float chamber gasket crushes in the ring groove of the float chamber to make it airtight, so it should be replaced with a new one when the float chamber is removed, whether for maintenance or setting. Reusing a gasket that has hardened over time will inevitably result in over-torquing when tightening, and may cause the mating surface of the float chamber and carburetor body to distort.

021eb-min.jpgThe crushed gasket (right) sandwiched between the float chamber and carburetor body is prone to fuel leaks from the mating surfaces due to reduced surface pressure as the contact area becomes sticky. The new gasket (left), on the other hand, has a round O-ring cross-section with a point of contact, which increases the surface pressure and ensures a good seal for the fuel in the float chamber.

022eb-min.jpgFrom STD to custom, each owner has his or her own way of enjoying the long-selling Yamaha SR400. In both cases, carburetor maintenance and settings are important to keep the motorcycle performing well. Judging how thick or thin the mixture is and how it changes with jet or needle modifications is not something that can be learned overnight, but with the Carburetor Fuel Tuning Kit, you can experience the settings step by step in a permutation combination.

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