The pilot screw
There are two types of “pilot” or idle mixture screws on VB carbs. Both are positioned downstream on the carb, that is, on the engine side, and therefore, though often called “fuel” screws actually meter idle air and fuel together, not just air as on earlier carburetors. The earliest VBs have no adjustment limiting flag, but shortly after production started, aluminum limiting flags appeared, at least in the US. These were super-glued on and thus easily removed using a professional grade soldering gun, and they must be removed to properly adjust these carburetors. The pilot screw’s tip can get stuck in the carb casting and break off as a result of rough handling. Repair in most cases means replacement of the carburetor body. All VB series carbs use the same coarse-threaded pilot screw (though some with flags and some without), the exact same pilot screw o-ring, and the same washer as well. The washers and o-rings are available from several sources. The correct pilot screw setting on all VB carbs is the same, 2 to 2 ½ turns from gently closed. Obviously, this is an initial setting and fine tuning from this is required. It is also different from the official Honda spec. If a bike exhibits a need for a larger setting than 2 ½ turns, this indicates an issue with the carburetor, most likely an idle circuit that is not completely clean. Also beware that a setting much larger than 2 ½ turns--say 3 turns--is never legitimate and comes very close to the screw’s spring losing most of its tension and the screw will then be subject to unscrewing itself and falling out of the carburetor. When rebuilding VB carbs care must be taken to dislodge the pilot screw o-ring which tends to stick in the carburetor casting. Having two o-rings in there as a result of a rebuild will make the engine idle noticeably unsteady.
Float bowls
There is more than one float bowl shape that fits on the VB carb--the CB750 and CBX for example use different bowls but use the same gasket. The gasket is available from many sources including K&L. All the VB series carbs have civilized float bowl drain systems in which the screw is opened and the fuel exits at the overflow nipple at the bottom of the float bowl. By contrast, earlier Keihins drained through holes in the screws themselves, an awful design that makes a mess with fuel pouring onto the engine. The o-ring found on VB series float bowl drain screws does not as many think seal the screw. It seals the outside of the float bowl body when the screw is opened, to eliminate fuel gushing out around the screw instead of through the float bowl’s overflow nipple. Later Keihins such as the VD series have the same screw but without the o-ring.
Overflow standpipes
VB series carbs have a universal fault, that of cracking their float bowl overflow standpipes. This cracked standpipe problem is well known in the carb rebuilding community. They are easily solder-repaired, unless the cracks are too wide. In that case aftermarket replacement standpipes are available and should be used. This cracking is so common that easily half of the VB carbs I see have it, and often I will see indentations beat into the side of a float bowl, the result of someone banging on the bowl in an effort to stop an overflow episode. Light tapping of a float bowl to stop an occasional overflow due to dust or water inside the bowl is a legitimate and commonly used recourse. But if it is not effective, other measures need to be employed. The float bowl should not accrue a myriad of pockmarks from repeated hammering.
The auto valve
Honda calls it the “auto fuel valve”. It is merely a vacuum-operated fuel on/off valve intended to eliminate the risk of hydrolocking an engine’s cylinder and the damage to the engine this can cause. Despite different shapes on different models, this valve does the same job in each case--it stops fuel flow when the engine is shut off. Eventually its internal rubber diaphragm will fail, almost always very gradually choking off fuel, and also usually leaking manifold vacuum, both of which will lean out the carburetors. More than half of the auto valve equipped VB carbs I rebuild arrive with bad valves. Some of the valves are rebuildable and some are not. A common error people make on installing these carburetors is to mix up or otherwise incorrectly install the vacuum and the vent hoses that come off the auto valve. Removing the auto valve to improve cold starting is a good thing to do because VB carbs’ float bowls evaporate their fuel within a few days when sitting and then the auto valve requires cranking of the engine to refill the float bowls, making starting take longer, the spark plugs subject to fouling, and the starter motor subject to more wear. When the auto valve is removed however, extreme caution should be exercised--the manual fuel valve (“petcock”) needs to be shut off each time the bike is parked.
The air cutoff valve
Almost all VB series carburetor sets have an air cutoff valve. This diaphragm-operated valve is plumbed into the idle circuit’s air bleed passage. All it does is momentarily close off that passage on engine deceleration. This results in a greatly-enriched air/fuel mixture, so that afterburn in the exhaust is eliminated. Afterburn can also happen due to other reasons, so this valve does not in every case solve that. Defeating the air cutoff valve was actually first recommended by Honda themselves in a publication directed at production roadracers. Defeating the air cutoff valve has the important benefit of eliminating it as a constant and expensive repair item. And it is very easy to do. Simply block off the valve’s access to intake vacuum. Defeating the valve has no other benefit. It will not improve performance. And, contrary to what is said on user forums, defeating the air cutoff valve does not automatically demand that the carburetor’s pilot screws be richened to compensate. The correct, greater-than-OEM-spec pilot screw setting should already have been performed irrespective of the air cutoff valve’s use or disuse, at the time of the bike’s sale or shortly after. Beware that carbs that have air cutoff valves on them cannot be chemically cleaned without first removing the air cutoff valve diaphragms. Every season I see a handful of carbs whose diaphragms are melted by someone spraying aerosol carb cleaner into the carburetor without remembering to first remove the diaphragm. This is especially noticable on DOHC four carbs whose diaphragms are not easy to get to without unracking the carbs.
Part 4
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