The “Mikuni TMR” and “Keihin FCR”. In the modernized EFI, It is a world-class carburetor that is enthusiastically supported. Yoshimura has set these two models MJN specification for the popular motorcycle model. It’s a high-performance carburetor that cannot be tasted with EFI is attractive and profound. The more you know, the more fun it is.
The fun of TMR and FCR carburetors which show excellent performance even in EFI era are consistent.
The world’s leading “Last Racing Carburetor,” Mikuni TMR and Keihin FCR, appeared around 1990, shortly before the EFI era. Furthermore, Yoshimura’s “MJN” (multiple jet nozzle), invented by Fujio Yoshimura, was brought evolution to the carburetors. In practical terms, the carburetor era is over, both in the race and on the street. However, in the world of carburetor vehicles cars and mini bikes, carburetors bring performance and enjoyment that EFI does not have. In such a carburetor world, TMR and FCR are rivals.
There are structural differences between TMR and FCR, but there is no clear performance difference. Since from old days, the bore diameter is odd for Keihin and even for Mikuni, but actually, there is no difference in power between FCR 39mm and TMR 40mm. It is chosen by design rather than that. The difference in riding quality is the difference in settings, so it cannot be said that it is the difference between the carburetor. It is the ultimate carburetor that overcomes the weaknesses of the carburetor until then. There will be no other precise carburetor like this so far, it’s fabulous, “Mr. Fujio said, the inventor of MJN. His father Pops Yoshimura had been fighting with many racing carburetors such as Keihin CR, Mikuni Smooth Bore, Mikuni TM, etc., so he put his passion to TMR and FCR of the high-performance carburetors.
Mr. Fujimura Yoshimura is a representative of Yoshimura Japan. Renowned as an engine tuner/engineer, he is also a carburetor specialist who has been handling Keihin CR (old type) since the 1960s.
Both TMR and FCR are box-shaped flat valves. The commercialization first started with FCR (downdraft in 1989, horizontal in 1991), and TMR that was the rival of FCR was released a little later (horizontal in 1993, the downdraft in 1995). Of course, in the race, the leading model has been used for some time. And, between these two racing motorcycle carburetors, Yoshimura announced MJN in 1992 and started marketing.
In terms of absolute performance, the carburetor cannot beat the EFI. But it is not the case that EFI has exceeded the carburetor in all performances. In such that part, if there are no injectors, the performance and characteristics like a carburetor will not be achieved. For motorcycle, this is not realistic given space and cost. In the current EFI, it is the only misdemeanor so as not to notice it. The carburetor is also good for users to enjoy the settings. It is possible to do things like carburetor setting even with a sub-computer (such as BAZZAZ handled by Yoshimura), but it’s not safe and it costs too. The price of the sub-computer is several ten thousand yen. It is a carburetor that inhales air-fuel mixture naturally as the engine requires by the intake negative pressure of the engine, and EFI that injects a programmed amount of fuel. It’s the difference between fuzzy and digital.
The carburetor is interesting. You can set it by yourself. And because the motorcycle is a hobby, Not only just riding, but the world will expand if there is fun to touch the motorcycle. Even in a race, it’s easy to handle after all. It does not make sense to have only the power. I used a large carburetor with a bore diameter of φ40mm to give power in the 4-cylinder 750cc era, the problem is easy to handle also at that time. The important thing is how is it to utilize the air mixture properly and make it inhale efficiently. MJN was born at that time, “compared to the jet needle type, the sweet spot of the setting is wider and easier to set, and the throttle work is also much easier. In the first place, TMR and FCR were so easy to treat compared to the carburetor until then, and it was shocking at the time of debut. In terms of settings, the FCR has a main air structure, and in a good sense, it is easy to handle and set.
On the other hand, in TMR, since the main air is blinded, the characteristics change in the setting of the pilot system, and the severity of the setting is a little, but it is easy to identify the difference in the setting. Therefore, in Yoshimura, the pilot air jet of the MJN specification TMR was changed to a pilot air screw like FCR from the middle to make setting easier (this had a great effect).
The carburetor really works well. Comparing to the old Z1 and CB750FOUR OEM carburetor the difference is obvious. However, even if it is a high-performance carburetor, each part wears and deteriorates. Therefore, Yoshimura would like to strengthen its after-sales service in the future. That may not be the case with EFI, but aftermarket parts like carburetors still have room for product development. And today’s overhaul demand is steadily increasing. The reason is that because we want our customer to use our carburetor comfortably for a long time.
The carburetor is really interesting” TMR and FCR. Especially, TMR has a backup of Yoshimura Carburetor Overhaul, so the “Last Racing Carburetor” attached to your motorcycle will stay alive for a long time. Rest assured.
Both TMR (left) and FCR are horizontally mounted on the rear of the engine (both directions are right is the engine side), but the placement of the throttle valve shows that TMR is vertical and FCR is slanted.
The back of the carburetor. There are two air systems line up under the bore. The main air is at the center and the pilot (slow) system is on the outside. TMR (left) is the main air blind (blocked) and the pilot system is a replaceable pilot air jet. FCR (right) is a structure that applies main air, and a slow system (the name of Keihin. Mikuni is pilot) is a variable air screw. Yoshimura’s TMR-MJN is a pilot air screw type. The projection in the bore is the nozzle of the acceleration pump.
The link is visible when the top cover of the carburetor is removed. Both the TMR (left) and the FCR (right) use the link to suspend the throttle valve. The link ratio is slightly different, and the FCR dares open at the beginning to make it easy to handle with a low throttle (Not controlling the throttle on the right-hand situation). Conversely, TMR emphasizes a direct feeling.
The throttle valve operates smoothly using bearings. The three FCRs (left) are mounted on the throttle valve side, and one takes advantage of no bearings to absorb rattling. TMR is a separate bearing structure in which rails with small bearings are mounted between the body and the throttle valve. FCR wears down to the carburetor body (when the rattling noises are loud) if you use it. However, since the TMR and the throttle valve are separate bodies, the carburetor body will not wear out. In addition, the wear goes fastly in town use than race use.
MJN specifications for FCR (left) and TMR (right). The cab top cover and float chamber drain are red with MJN specifications. In FCR-MJN, the float chamber drain is longer than STD. This is due to the MJN pipe being longer than the jet needle (the fuel nozzle is down).
Original Source[ Bike Bros ] (*Japanese)
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