® Victor bravo part 5: Keihin VB series primer


The accelerator pump
Most VB series carbs have an accelerator pump. And it works better than those found on other kinds of powersports carburetors--even racing carbs such as the famous Mikuni RS/HS. However, it has one glaring fault: its discharge nozzle clogs up easily. I have found a guitar string to be the best solution. I have a piece clamped in a pin vise, which is just a handle to hold very small drills. The pin vise is extended with a length of welding rod for better reach into the carburetor. After opening both the throttle and choke plates, I shoot a flashlight down the slide bore while reaching in with the tool to the accelerator pump's upright nozzle. Once in the discharge orifice, I rotate the tool to auger the hole a bit, which dislodges any varnish or rust that is packed in there. This is very effective. After trying hydraulic methods, heat, and more, this technique has become the one I always use, when needed, on all of the VB series carburetors I rebuild. The VB carb’s accelerator pump is also adjustable just as with racing carbs. Adjustments can be made for when it starts working and when it stops.

Float level
A common misunderstanding is that a carburetor’s float level is what it is to prevent fuel overflow. This is not true. Float level is a mixture determining feature just as the main jet size is. The level chosen by the manufacturer exposes to all the circuits a given amount of fuel. It has nothing to do with whether the fuel will overflow. If float height was causing your overflow, you would have already long since noticed how badly the bike ran and that would be your main concern. There are many things that can cause carburetor overflow, but float level is at the bottom of the list. It is fruitless to try to solve overflow by changing the float level. Not only is float level the least likely cause, in most cases modifying the specifications of a carburetor to solve a basic functional issue simply makes no sense.

Slide lift
The ratio between throttle movement and slide movement is not directly proportional in a constant velocity carburetor. Incidentally, the enlargement of the slide lift holes at the bottom of the slide, as many jet kits suggest, is intended to make this movement somewhat closer to linear, for racing purposes. In the VB carb, as in most CVs, the slide does not even begin to lift until around one-quarter throttle. This is why CV slides are vertically positioned so high in the carburetor bore. Many folks, unaware of this, often focus on the slide in connection with carburetion difficulties--polishing it, fretting about its condition, etc.--when in reality the slide is much less active in a CV carb than it is in a variable venturi carburetor, far less than people assume. Of course, no hard and fast rule can be applied to its precise movement because that movement is dependent very much on engine load. The combination of low gear and high rpm resulting in very low loading produces almost no slide movement, while higher gear ratios and low rpm combine to produce very high engine loading and maximum slide movement. The engine really does operate the VB carburetor. In connection with this, forums and other voices all over the Internet wrongly attribute carburetion problems after removing the air filter box to changes in slide lift proportionality. This is untrue. The reason carburetion suffers so much when the air filter is removed is simple air/fuel mixture. Honda’s air filters are extremely restrictive. So much so that removing the airbox on a 1979 CBX1000 necessitates a ten-sizes change in the main jet. Yes, ten sizes. It has nothing to do with slide movement and even less to do with the exhaust. The reason Honda’s air filter systems are so restrictive has to do with federal sound regulations that affect manufacturer importation.

Slide rings
As mentioned earlier, the plastic rings beneath the slides on VB carburetors are not gaskets. They are cushions. The VB’s vacuum top seals metal to metal, exactly like the vacuum top on the DOHC 450. The correct orientation for the ring is with the ridge facing upward. The ridge is a spacing element, holding the slide up vertically in the carb bore at an engineered height. Every model VB carb has a different height ring holding its slide at a unique height. If the ring is installed upside-down--and I see many in my work that are--the ring will hold the slide at a different height than intended. This can affect starting, as the reason for each VB carb having a different height is the height works with the choke during starting. Having the ring in upside-down will also deform it when the vacuum top is tightened down. It doesn’t fit properly upside-down and when the vacuum top is bolted on, it presses the ring into an unnatural shape. This cannot be repaired. The ring must be replaced.

Throttle plates
Also mentioned earlier, the VB throttle shaft and plate do not have part numbers. Nor do the felt seals that surround the shaft. It is unwise to disturb any of these parts because the throttle shaft and plate don’t reassemble in an intuitive way. The throttle plate is very delicate. Its bevel-machined edge is easily damaged. The plate is actually larger than the carb throat because at rest it seals the carburetor at an angle. The throttle plate on a CB1100F carburetor for example is exactly 1mm larger in diameter than the carburetor’s throat. The throttle plate screws are swaged to prevent unscrewing, and of course new screws when installed need to have thread locker on them for the same purpose. It is bad practice to reuse the screws, as that will potentially damage the delicate threads in the choke shafts.


Last updated January 2025
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