The primary main circuit found in most VB series Keihin carburetors is a holdover from Honda's 1960s CV carbed models, the CB350 twin and the CB450 twin. The 3-jet CV of course continued into the early 1980s. Honda's CBX had the three-jet system only its first year, while the CB900 had it throughout its production, and of course the CB750 as well but not the CB1100F. I am speaking of and know well only Honda's US models.

The primary circuit is a transition circuit. Its range of influence is roughly 1/8 throttle. This is something of a misnomer as CV carbs unlike mechanical carbs are not analyzed by throttle position but rather by engine load vs. rpm. Mark Dobek of Dynojet did much to make the public aware of this distinction. I am sure you know that CV carbs' slides do not lift linearly in step with the throttle handgrip. The 3-jet carb's advantage over the two-jet system is that it is more flexible, that is, when performance tuning is in view, it is better to have a 3-jet CV than it is to have a 2-jet, and in fact the CBX community is aware that when doing intake and exhaust modifications to a CBX a 3-jet 1979 carb is preferred. It is more tunable, easier to make accommodate individual air filters, for example.

The primary main circuit is not on center with the slide. It is offset from center, and note, is linked to and feeds the idle circuit. Again, 1960s tech. The emulsion tube beneath the primary main is not a metering part. Replacement primary tubes suitable for all the range of VB carbs having primary circuits are all one spec in terms of hole sizes and placement, despite variation in the primary tubes from the factory on the VB carbs wherein they are found: CB250, CB400/450, CX500, CX650, CB750, CB900, CB1000, GL1100 and CBX. The aftermarket tubes work well. I have used hundreds of them in rebuilds.

You are probably aware that Honda recommended in their racing parts kit literature for the first-gen CB750 DOHCs the defeating of the primary circuit, essentially converting the 750 carb to CB1100F-like specs, that is, making the racing 750 a 2-jet carb. This was done by blocking off the circuit with a screw and richening the main circuit. This is instructive as it points to the overlap nature of the primary circuit, seeing as how the main could compensate, and illustrates Keihin's penchant for designing carburetors with many small circuits as opposed to Mikuni's way of building carbs with fewer larger ones. At the same time the factory race kit instructions endorsed defeating the air cutoff system as well using a tiny steel ball. My take on both of these "racing-only" mods is that their purpose was to reduce fuel consumption for long races, particularly endurance type. There is no performance result in any case, contrary to what some forums want people to believe.

The primary tube often gets stuck in the carburetor and this is due to a defect in manufacturing. The carb casting's threads runs out short of the installed depth, making the tube kind of force-thread during installation. To avoid this, I always thread them in very lightly, without any special tightening. Sometimes I even run a tap in before installing. To remove a stuck tube the best approach is to simply drill out the short threaded portion. It is then easily removed.

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