® Victor bravo part 2: Keihin VB series primer


Jetting and circuits
The earliest Honda VB series carbs are known as 3-jet carbs, because their tunable circuits are: idle, primary main, and secondary main. A holdover from Honda’s 1960s bikes, Honda eventually phased out the 3-jet system in favor of the simpler 2-jet carb early in the VB carb family’s production timeline. The primary main jet in the 3-jet carb, despite its name, is actually a midrange jet. This makes the 3-jet carb easier to modify to suit engine mods, such as those often typically applied to the CBX1000 and first-generation DOHC fours. All VB carbs tuck their air bleeds under the slide, another interesting feature. VB carb idle jets started out as pressed-in but in the last few years of production conventional screw-in idle jets appeared. By contrast, Keihin’s PD series carbs that started at almost the same time as the VB series never advanced to removeable idle jets from their pressed-in idle jet design. Three-jet VB carbs have a rubber plug over the idle jet, in the idle jet tower. This is because the idle jet isn’t fed straight from the float bowl. Instead, the idle jet is fed from the primary main jet tower. There is a lateral hole between the two jet towers that allows this. There seems to be some confusion about the pressed-in idle jets. Honda’s PD series carbs had normal length (long-headed) pressed-in idle jets. Honda’s VB series carbs on the other hand used the shorter-headed version of the same pressed-in idle jet. Otherwise they’re the same. Aftermarket jet suppliers have both and are the only sources for new idle jets. Honda never sold them. The pressed-in idle jets are easy to remove and can even be reused, which is preferable due to the inconsistent quality of aftermarket idle jets. And despite statements found online, there is no benefit to putting in larger idle jets, not even to compensate for ethanol, and there is no inherent leanness in the idle circuit on VB carbs. All VB series Keihins employ Honda’s classic –393 series, round, hundredth-millimeter sized main jets, the jets found in most (though not all) Honda carburetors.

Floats and float valves
The CBX1000 version of the VB carburetor has removable float valve seats, but most VB carbs do not. The earliest VBs also have adjustable floats, while the later ones do not. The adjustable floats use the –413 float valve, the non-adjustable the –382 float valve. The one-piece, hollow plastic non-adjustable floats rarely take on gasoline, but they do often warp, resulting in their not accurately doing their job and must occasionally be replaced. The conventionally non-removeable float valve seats in some VB models are actually removable using machinist’s techniques and replacements are available from Keyster. Don’t abrasive-clean the float valve seat. Chemical cleaning is all that it needs and rubbing even a fine abrasive into it will change its shape, potentially degrading sealing. Aftermarket float valves are uniformly bad, and most of those available for the nonadjustable float model VBs are slightly longer than OEM, resulting in a leaner-working carburetor.

The primary tube
The 3-jet version of the VB carburetor has an emulsion tube underneath the primary main jet. Often difficult to remove, some have suggested screw extractors and other desperate removal measures. Stay away from those methods to avoid grief. Experienced carburetor rebuilders know that when encountering a stuck tube (which happens often) to simply drill out the small number of threads in the tube and it will fall right out. Honda long ago discontinued this part, but the aftermarket Keyster item is good quality. A vast number of them are in very effective use all over the world in virtually every iteration of the 3-jet VB carb family and have been for scores of years now. Though the size and number of holes in the original tube vary on different models of primary tube equipped VB carbs, the Keyster part substitutes for all of them admirably. This is possible the primary tube is not a metering part but an emulsifying part.

The jet needle and needle jet
As with the vacuum top and slide, Honda sold the jet needle and needle jet as a set. Not matched, just sold together. Most of the VB series carbs sold in the US have stainless steel needle jets while the jet needles are hard-annodized aluminum or more likely another metal, though there are exceptions on some models. The needles, due to their special material and floating attachment in the slide, and very unlike their pre-1978 Keihin counterparts, are not conventionally adjustable and do not wear. Vintage jet needle/needle jet sets are unavailable from Honda now, making the subsitution of these parts with cheaply-made, poorly metering, softer brass aftermarket carb kit parts extremely unfortunate. And costly to correct--you must buy a second set of carburetors and mine its parts. The engine modifier’s expedient of shimming the jet needle to accomodate engine modifications is seldom beneficial on VB carbs, particularly the 3-jet version, because midrange tuning in that carb is better facilitated at the third jet. Furthermore, when shimming the needle, where appropriate, it is important that a uniquely-sized washer be used, and the needle holding screw should be also modified at the same time so the needle retains its “floating” characteristic. These two important precautions are often overlooked by the home mechanic.

The choke
All VB series Honda carburetors are very retro in one sense--they all employ an old-school choke type starting system. Later Keihin carbs such as the VD, VE, and VP series have dedicated starting circuits. The VB’s more archaic choke system means that the idle jet is what the engine starts on, making the idle jet twice as critical as it would otherwise be. Therefore using a fuel stabilizer such as Sta-Bil helps ease of starting on the VB family of carbs, in addition to its general carburetor maintenance benefit.

The fast idle mechanism
VB Keihins include a fast idle linkage system that nudges the throttle bellcrank slightly when the choke is applied. This fast idle system is a huge advantage over Keihin’s earlier variable venturi carburetors, most of which have no fast idle feature. The VB fast idle system is however somewhat quirky in that it can cause some unfortunate issues with the engine’s idle when either the choke cable is not very carefully installed to have plenty of slack, or the fast idle system’s linkage is out of adjustment. Yes, it is adjustable. Beware.

Part 3


Last updated January 2025
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