Not just for genuine carbs. Keihin FCR and Mikuni TMR also available: Keystar's fuel adjustment kits.

【PR】Kishida Precision Industry By Akira Kurita

Keyster, a manufacturer specializing in carburetor parts that sells fuel adjustment kits for over 500 models from mopeds to big bikes over 1000cc, is trusted not only by users of discontinued models but also by specialty shops for classic motorcycles. The biggest feature of the fuel adjustment kit is that it allows for setting changes to genuine carburetors. On the other hand, they also develop fuel adjustment kits for racing carburetors such as Keihin CR Special, FCR, and Mikuni TMR. Even for special carburetors designed for setting, their fuel adjustment kits incorporate unique features.

Fuel Adjustment Kits Earning High Praise from Discontinued Model Users

Many users are helped by fuel adjustment kits, which enable setting changes for rare genuine carburetors no longer available new, and are also useful for overhauls and maintenance.

By independently developing carburetor parts, Keyster sells fuel adjustment kits compatible with over 500 models of Japanese motorcycles. As is well known among enthusiasts of discontinued and classic models, the characteristic feature of a fuel adjustment kit is its ability to change the settings of genuine carburetors.

Genuine carburetors are set by motorcycle manufacturers during new model development to achieve the best balance with the normal engine, normal muffler, and normal intake system. The sizes of the pilot jets, main jets, and jet needles installed at that time are fixed.

However, if customizations are made, such as replacing the muffler, removing the air cleaner box, or changing the internal specifications of the engine, the genuine carburetor with normal settings may not perform optimally. In such cases, a fuel adjustment kit, which includes jets and needles of different sizes to allow for fuel adjustment centered around the genuine size, becomes invaluable.

FCR Fuel Adjustment Kits Sold with Standard Settings for Major Models

An example of an FCR fuel adjustment kit. FCR carburetors are general-purpose, but Keyster's fuel adjustment kits are characterized by being developed with a base setting tied to the venturi diameter and model. Therefore, for example, there are three lineups for the Zephyr 1100 kit: for φ37, φ39, and φ41.

When it comes to carburetor settings, people tend to focus on the main jet. However, in actual riding (especially on public roads), the setting of the pilot jet, which controls the low throttle opening range, is crucial. This is because the difficulty of opening the throttle increases with larger displacement engines.

The FCR fuel adjustment kit includes four jet needles. The straight diameter of the needle is effective around 1/8 to 1/4 throttle opening.

While genuine carburetors for production vehicles have a "base setting," the appeal and characteristic of general-purpose racing carburetors lie in their ability to be set "as desired." Even with the same venturi diameter, the settings differ depending on whether the engine is 400cc or 750cc, so it is up to the user to determine the sizes of the jets and needles.

However, simply increasing the hole diameter of the jet to increase power is not the whole story. Setting is determined by how much air-fuel mixture the engine can ingest, and it is necessary to prepare multiple sizes of jets and needles and refine the settings while observing the color of the spark plug.

One notable aspect of the Keihin FCR fuel adjustment kits developed by Keyster is that, similar to the fuel adjustment kits for genuine carburetors, the settings are pre-determined for representative models equipped with FCR carburetors.

For example, even with a φ37 horizontal type, the jet and needle set sizes differ for the CB1100F, GSX1100S, and ZRX1200R. Keyster determines these based on a comprehensive review of various information. Furthermore, settings can be made with three sizes of pilot jets, six sizes of main jets, and four sizes of jet needles.

For racing carburetors like the FCR, genuine jets and needles can be purchased individually, one size at a time. However, it is not guaranteed that a single size will result in satisfactory settings. In such situations, the convenience and flexibility of an FCR fuel adjustment kit, which comes with multiple jets and needles pre-assembled, truly shine.

Full Overhaul Possible with Gaskets for Non-Disassemblable Parts

The two-story body is fixed with screws and adhesive and is not designed for disassembly. However, to replace the gasket installed here, it must be disassembled, albeit with some force.

There is an irregularly shaped gasket at the bottom of the body and in the core of the throttle valve. Although the image shows it is mostly undamaged, using chemicals or parts cleaners that are incompatible with rubber after long-term storage can cause swelling, deformation, and blockage of passages.

The gasket independently produced by Keyster is a part that even carburetor manufacturers do not offer. Many FCRs are saved by this.

The appeal of FCR fuel adjustment kits extends beyond just the combination of jets and needles. Similar to fuel adjustment kits for genuine carburetors, they also include parts that are replaced during overhauls and maintenance, such as gaskets for the top cover and float chamber, O-rings for the fuel joint, needle valves, and accelerator pump diaphragms.

Particularly noteworthy is the "intermediate body gasket." FCR carburetors have a two-story body structure, with a core inserted into the throttle valve. Therefore, a special-shaped gasket is placed between the body and the core to prevent gasoline and air leaks from the mating surfaces.

This part is considered non-disassemblable by carburetor manufacturers, and there is no gasket part number available. However, due to the effects of chemicals used in maintenance and internal cleaning, or due to aging, the gasket can deteriorate and cause problems.

Keyster has independently created an intermediate body gasket, making it possible to perform a full overhaul, including gasket replacement, by purchasing an FCR fuel adjustment kit. Disassembling a body designed for non-disassembly requires considerable skill in motorcycle maintenance and carburetor tuning. However, the ability to obtain parts that are generally unavailable makes this kit highly valuable.

FCR "Step Wear" Solved by FCR Rebirth Kit

The FCR Rebirth Kit repairs wear inside the body. The idea of covering the worn part of the body with a guide rail is groundbreaking. In addition to stainless steel guide rails, it also includes bearings for the throttle valve and dedicated rollers. The Rebirth Kit is not sold separately and must be purchased as a set with the FCR fuel adjustment kit.

The idea of inserting a U-shaped guide rail into the body is simple and unique.

Another unique and distinctive FCR-related product alongside the fuel adjustment kit is the "FCR Rebirth Kit." FCR carburetors have a characteristic where the inside of the body wears down due to the pulsations of the engine's intake vacuum. Specifically, around idling, where the intake vacuum is strongest, the throttle valve roller presses against the body, causing step wear and making throttle operation heavier.

In response to this symptom, Keyster devised a method to cover the step wear with a 0.1mm thick stainless steel guide rail and commercialized it as the FCR Rebirth Kit, which includes bearings and rollers for the throttle valve. Installing this kit improves the throttle valve's movement, making it lighter and easier to operate, and it has been highly praised by many users.

TMR and CR Special Fuel Adjustment Kits Allow Users to Select Jets and Needles

The characteristic of the fuel adjustment kit, which includes consumables for maintenance and overhauls in addition to setting elements, is also inherited by the TMR fuel adjustment kit.

Unlike the FCR version, the TMR fuel adjustment kit allows the user to specify the sizes and combinations of jets and needles at the time of purchase.

For maintenance of carburetors that have been used for a long time, it is desirable to replace the needle valve with wear marks from the valve seat. The fuel adjustment kit includes the needle valve and valve seat, which can prevent unexpected overflows.

The O-ring of the fuel joint may not cause problems while immersed in gasoline, but it often cracks or becomes brittle after being removed due to long-term storage. It's disappointing when it starts leaking drop by drop after setting, but the O-ring in the kit can prevent this maintenance issue.

Another special carburetor that adopts a flat valve, similar to the Keihin FCR, is the Mikuni TMR. Keyster also develops fuel adjustment kits for TMR, and it's worth noting that the settings for these are much simpler compared to the FCR fuel adjustment kits.

As mentioned earlier, the FCR fuel adjustment kits have base settings determined by the model and bore size. In contrast, the TMR settings are divided into two sets: φ28-35 for small bodies and φ36-41 for large bodies. Both are for single-cylinder use (one carburetor), meaning you need to purchase two sets for a twin-cylinder or four sets for a four-cylinder.

A further characteristic is that the user can specify all the setting elements – pilot jet, main jet, jet needle, and needle jet – at the time of ordering. While ordering excessively different sizes carries the risk of the engine not running properly, Mikuni provides a basic setting table for each body size, which serves as a reference. By choosing the setting elements based on this table, the risk of a major miscalculation is reduced.

Furthermore, the fuel adjustment kit includes three sizes of pilot jets and six sizes of main jets, allowing for setting changes that accommodate the engine and muffler specifications.

This made-to-order fuel adjustment kit is also applied to Keihin CR Special. In addition to the distinction between small bodies (φ26-33) and large bodies (φ35-39), there are four sets for single-cylinder and four-cylinder applications. However, the process of specifying the sizes of the pilot jet, main jet, and jet needle is the same as for the TMR fuel adjustment kit.

Additionally, both TMR and CR Special kits include gaskets for the top cover and float chamber, as well as gaskets for the core, needle valves, and valve seats, making them valuable for maintenance and overhauls.

The situation for aftermarket special carburetors is as precarious as the disappearance of genuine carburetors. Keyster's fuel adjustment kits are expected to be of great help in enjoying these valuable carburetors for a long time to come.

The fuel adjustment kit for Keihin CR Special, compatible with discontinued models from the 1970s, allows users to specify jet and needle sizes, similar to the TMR version. For setting changes, it is advisable to determine whether you want to lean or enrich the mixture from the current jetting and then decide on the order size.

The main jet includes two leaner sizes and three richer sizes relative to the center size. The pilot jet includes one richer and one leaner size each relative to the center size. The jet needle also includes one richer and one leaner size each, based on the center size.

The top cover gasket and the core gasket (left, round tube) are made of paper, while the float chamber gasket is made of rubber. The core is screwed to the bottom of the body, ensuring a smooth bore without irregularities on the inner surface of the venturi when the throttle valve is fully open.