The seemingly extra VB series carburetor circuit, the "primary main" jet found on the 400 and 450 twins and the CX500, 750, 900 and CBX, is a holdover from Honda's 1960s CV-carbed models, the CB350 twin and the DOHC CB450 twin. Its name is misleading since its role is dissimilar to the carburetor's main jet (whose name is the "secondary main" in the 3-jet carburetor). Rather, the primary main jet is functionally far lower in the engine's rpm range, between the idle jet and the jet needle. That is, nowhere near the larger secondary main jet's range. Its physical placement in the carburetor is off to the side of the secondary main jet, adjacent to the idle jet, and oddly, it actually feeds the idle jet. That is, the idle jet is capped and the fuel entering it comes from the primary main jet. Again, this is 1960s Keihin/Honda tech. Though Honda has run out of replacements for the emulsion tube found beneath the primary main jet, new tubes are available on the aftermarket.

The primary main tube needs to be removed during a carburetor rebuild. Unfortunately, it often gets stuck in the carburetor, frustrating many DIY mechanics. This is due to a defect in manufacturing in which the threads in the carb casting do not perfectly match the threads of the tube. To avoid tube seizure, thread it in very gently. That will make future removal easier. To remove a stuck tube the best tactic is to simply drill out the very short threaded portion. It is then easily removed. No other approach is as effective.

Honda recommended in their racing parts kit literature for the first-gen CB750 DOHCs the defeating of the primary main circuit, converting the 750 from a 3-jet to a 2-jet carb. This was done by blocking off the primary circuit with a screw. The modification gives the 750 carb CB1100F-like fuel circuits, making tuning for racing easier. There is no performance benefit from the modification, despite forum rhetoric to the contrary.

For more on carburetors in general, check out my carburetor series.

Part 7