® Victor bravo part 4: Keihin VB series primer


Pilot screw troubles
The VB series Keihin carburetor comes in for more than its share of pilot screw problems. The most common is when the screw threads seize in the carburetor body. Oftentimes, simply carefully heating the body with a small torch will free the screw up and it can be unscrewed. Another symptom is when the head of the screw is mangled or broken or the screwdriver slot torn up. The most expedient and a very effective remedy is to Dremel a new slot in the screw and by that it can usually be removed, and a new replacement installed. And then there is the worst-case situation, when the pointy tip of the pilot screw has broken off and is stuck in the carb body. This is a heartbreaking thing. The idle discharge outlet is where the tip is buried, and it can often be seen poking out there. The challenge is to remove the tip without augering or otherwise deforming that outlet. This is hard to do. I have successfully heated the carb body and using a pushpin or other similarly tiny steel pin--even some specially-made tools--pushed the errant tip out from inside the carburetor throat. But to date this has worked for me only about thirty percent of the time, with the other times necessitating carb body replacement. Take this to heart. Be very careful, i.e. gentle, when adjusting the VB carb pilot screws, and when in-hand during a rebuild inspect them carefully for bent tips which are the most in danger of seizing.

Float bowl venting
Many do not recognize the float bowl venting on the VB carburetor. On a few models such as those on the CX500, the venting is the same as on the older variable venturi Keihins, via small brass nubs on the sides of the carburetor body. These must be vetted during a rebuild. The more common VB carb bowl venting configuration however is via cast-in towers on the carbs’ sides which correspond to short rubber hoses so that all of the vents can be joined together. One thing must be realized however. At the center of the carb assembly, one of these joinings must include a hose tee and hose coming out to atmosphere. If this is overlooked and a simple hose is substituted, the float bowls will not vent and major fueling issues will result.

Marking carb bodies
When you see scratches or numbering applied to VB carb castings, this is a sign that that someone did not have a complete understanding of these carburetors. No matter the Honda bike model, each of the VB carbs in a multicylinder assembly is unique. They are not difficult to tell apart. The way professionals arrange the bodies prior to reracking is we identify the outer bodies first--they are easily identifed by the lack of outboard bowl venting towers and by the steel welch plugs at the throttle shafts. Then the other carbs are identified by the throttle bellcrank and related linkages.

Pipes and tees
One of the most vulnerable pieces in the VB series carburetor is its aluminum accelerator pump transfer pipes, on those model VBs having the accelerator pump. Age-hardened o-rings combined with even just slight carb body corrosion makes these pipes fairly difficult to remove. Thus the tubes can bend and even crack at the grooves machined into them for the o-rings upon assembly or disassembly. I inspect them carefully and have found a number that were cracked. The VB carbs’ fuel tees are also somewhat vulnerable. Simply brass parts that have been silver-solder assembled, rough handling can easily distort them. And of course these parts are expensive to replace because they have to come off other carburetors.

Buna versus Viton o-rings
The trend right now of course is to use Viton on everything from valve guide seals to baby rattles. Seriously, it is a shame that VB series Keihin carburetors are caught up in this nonsense. Though Honda used buna o-rings in production, and these are observed to have lasted almost fifty years up to the present, the Internet “experts” insist that only Viton o-rings are to be used by a carburetor rebuilder, citing ad nauseam their improved resistance to ethanol. In the first place, ethanol is a non-issue. And although the superiority of Viton is questionable, I use it because my customers expect it, not because Viton is demonstrably better than buna. In fact, Viton presents a number of challenges. It is up to three times as expensive, the material has a more limited size availability, it is rated with a much firmer hardness spec--making carburetor assembly and disassembly a little more difficult--and recently reports have begun surfacing that Viton is--surprise!--actually less resistant to gasoline than is buna, not more. Who knew? But it figures.

Floats
There are two kinds of floats used in VB carburetors, solid and hollow. Many of my customers ask about float replacement. Unlike the hollow brass items of Keihin’s earlier carburetors that took on fuel, the VB series hollow plastic floats have never been known to do that and are thus the least likely to need replacement. And the earliest solid ones cannot by design take on fuel and don’t on that count ever need replacement.

Rebuild kits
The factory never offered rebuild kits, instead listing in the parts book “packing sets”--basically carburetor gasket sets. Of course Honda also sold the jet needle and needle jet, fuel jets, diaphragms, float valves, and more, but each of these were separately marketed. They were not combined into kits. Aftermarket rebuild kits for VB series carburetors have many faults. The worst of them is that they encourage the replacement of the now universally unavailable factory needle jet and jet needle with improperly metering, poorly made substitutes. Other gotchas are aluminum pilot screw washers and overly-long pilot screw springs--both of which cause the pilot screw o-rings to not seal properly. Don’t underappreciate this. The idle circuit is the most sensitive, vulnerable circuit in the VB carburetor. Kits also contain fuel jets that are sized incorrectly, and float valves that are very cheaply made and fail to seal consistently.


Last updated January 2025
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