The Constant Velocity Carburetor is the OEM carburetor in the Honda Ruckus. It is made by the Japanese vehicle engine management company, Keihin, who has been in business since December of 1956. It started out as an automobile parts repair and processing facility and it wasn’t until September of 1957, when they started producing carburetors for the Honda Dream motorcycle and Fuji Heavy Industries Rabbit scooter. The carburetor model for the Ruckus is the NVK, but does use Main Jets for the FCR Carburetor. The FCR is a variable Venturi carburetor and not the same as the NVK, it is just that the Main Jets are. The Slow Jets come from a DVK Carburetor that is in Kawasaki V-Twin ATVs like the KFX-700 V-Force, which is a CV Carburetor and it may be a deviation of the Keihin CVK. It, however, does not use the same Main Jet as the FCR or the NVK.
Tuning the Ruckus carburetor is the same for any CV, Variable Venturi, or Fixed Venturi carburetor; you use the Plug Chop method and change your air/fuel mixture screw and main and pilot jets to tune accordingly. The only issue that arises is the difficulty to procure main jets at the time of this article. Keihin, for a reason not given to us, has not produced enough jets to keep up with demand. It is entirely possible that the main cause is a switch by many motorcycle manufacturers to fuel injection. As stated before, Keihin is an engine management company; which means they also manufacture ECUs, fuel injectors, and other EFI (Electronic Fuel Injection) components. With the Metropolitan and Zoomer, the overseas name for the Ruckus, becoming Fuel Injected bikes, the demand for jets for the NVK carburetor is down in demand except for here in the North American Continent where the Ruckus is still a carbureted bike.
So, what can be done? There isn’t much, sadly I must report. Until those jets are produced by Keihin or another aftermarket manufacturer, there is not solution except custom modification. This means finding ways to shim the needle and/or opening or closing the main jet for the perfect fueling. Shimming the needle wouldn’t be too difficult and is easily reversible; however modifying the main jet would not be easily reversible. While we do not recommend, and highly request that you do not, do this modification, you can use solder to close up a main jet and drill it out to enlarge it. Again, we do not recommend this modification as once you perform it, you will never get the jets back to the way they were. Shimming the needle is easy and can be undone; the problem lies with finding a shim thin enough to work. Brass washers are very thin and could work, but I have no data to confirm this other than knowing this is how Harley Davidson CV carburetors are modified. They also use a Keihin CV carburetor, but the CVK is not the same as the NVK.