Keihin’s somewhat unique VB ("victor bravo") series carbs are found on the (non-U.S. model) SOHC 250 twin; the SOHC 400 twin; the SOHC 450 twin; the 500 single; the 500 and 650 transverse V-Twins; the 1982-83 CB650; the first-gen DOHC 750cc, 900cc, 1000cc and 1100 inline fours; the CBX1000; and the GL1100. Many on user forums speak of the VB model Keihin as difficult to service and adjust and cite this as why they chose aftermarket high performance carburetors. However, despite little love on forums, the VB carb is not actually a difficult carburetor to work on, it does not lack performance--in fact it can be tuned to really perform well--and aftermarket carburetors are not as user-friendly in the everyday street environment as many think.
Vacuum top and slide
The VB family is characterized by its labyrinth type, non-diaphragm slide and its distinguishing can-of-peaches like vacuum top. These vacuum tops seal to their carb bodies metal-to-metal. Because of this, the top should never be plated, painted, or powdercoated. Rough handling that results in dents in the top can also seriously affect carburetor performance, as the resulting bulge inside the top will interfere with the close-toleranced labyrinth area’s movement. Nearly every different model of VB carb has a different thickness slide ring. This clues us to the fact that the ring plays a functional role beyond mere cushioning, i.e. that of tuning. It is not a gasket. The vacuum tops on all VB series carbs are interchangeable despite having different part numbers. The tops are sold only with the slide, but it is the slides that actually vary. The top and slide are not a matched pair. And like the tops, the slides are also interchangeable, varying only in their being updated from a cutaway design to a non-cutaway design a short time after production, and in the number and size of the negative pressure access holes at the bottom. Neither of these two specifications can be detected on a running bike and thus will not keep a slide from one model being fitted on a different model’s VB carb. The spring that works with the slide also varies from model to model--it like the slide ring also affects tuning--with every single model VB carburetor. The slide’s anti-topping plastic buttons vary almost as much. For example, the CBX1000’s four-year model range has two different buttons, and those are very different from those on the first-gen DOHC four carbs, and different again on the very similar appearing CX500 carb. The VB’s top and slide are very sensitive to crankcase-originating fumes that result in grunge deposits, making the maintenance of the crankcase recirculation systems on these bikes critical. The bronze bore in the top should be carefully cleaned and the slides also, especially around their chromed shaft. But not too aggressively--i.e. not with sandpaper.
Different size VB carbs
In a peculiar departure from traditional motorcycle carburetor engineering, the sizing of VB series constant velocity carbs centers on the carburetor throat, not on the venturi. That is, all VB carbs have the same diameter venturi, though the expected differences in throat and throttle plate size commensurate with their bikes’ engine sizes. This seems crude. And this is why all VB carbs’ slides are the same diameter and are interchangeable. The throttle plates are made of thickish aluminum and sport finely-machined, angled edges that enable them to virtually seal tight in the bore. Because of this delicate construction, nothing should ever be dragged beneath the throttle butterfly. No wire, no drill bits--nothing. The throttle plates do not have a part number, so replacements have to come from another carburetor.
Throttle shafts
VB carb throttle shafts and plates are vulnerable in other ways as well. Most people should not remove them, even during a carburetor rebuild. Even if you don’t fiddle with them, the throttle plates on Keihin VB carburetors are known to shift in place over time, resulting in idle unevenness, throttle stiffness, and other issues. Part of a rebuild is inspecting them for this and realigning them as needed. And any time the throttle shaft screws are loosened or removed, new screws should be installed and thread locker used.
Bronze bushes
Keihin VB carbs have pressed-in bronze bushings supporting the throttle shafts. These bushings do not require maintenance and they are not available as separate parts. They also have never been observed to wear appreciably. Some Suzuki carbs by contrast lack bushings and do wear appreciably, and Honda fuel injection throttle bodies often lack the bushes also.
Felt seals
The felt seals that protect these shafts and bushings are just that, seals against dust and contaminants. They are not seals against vacuum. Consequently, there is no performance symptom that can be solved by replacing them.
Identification numbers
Keihin VB series carbs are identified by a code marked on the casting. This starts with the all-important family moniker, “VB”, cast into the side of the body. This identification is augmented by characters stamped immediately above the float bowl. The first two stamped characters, as in “21” or “25” or “42”, tell us the motorcycle model. For example, “42” is a first-gen DOHC 750, “60” is a US-model CBX1000, “43” a first-CB900. The third stamped character—on VB carbs always a letter—signifies a model variation within a given engine displacement, an economy model versus a sport model, for example, and is sometimes accompanied by a very slight variation in jetting. The fourth stamped character is the least significant and typifies an even less important change such as the idle jet being pressed-in versus threaded-in, as in the case of the aforementioned DOHC 750, 900 and CBX1000. Stamped characters beyond the fourth are unimportant to the end user. In regard to all VB series Keihins, it is possible and has proved effective to, if necessary, mix carbs into the same rack that have different codes as long as the first two stamped characters are identical and the throat sizes and equipment specs such as air cutoff valve and accelerator pump are also identical. This has been done for ages with no ill result.
Part 2
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