Constant velocity (or vacuum) carburetors are a significant improvement over traditional carburetors. A reaction is created from the vacuum inside the intake manifold and not merely upon throttle opening.
This type of carburetor prevents the engine from blocking at low RPMs should you violently hit the throttle. It's mainly used on ATVs and most on-road motorcycles.
The CV carburetor incorporates a vacuum-operated slide that varies the venturi size within the carburetor, thus maintaining a constant velocity. The slide also holds a needle that, when lifted by the opening slide, varies the amount of atomized fuel delivered. A small, precisely-calibrated orifice (often called "jet") under the slide creates this vaccum.
It's almost like drinking from a straw. The drink goes up in the straw when you suck on it, right? The same phenomenon happens when the engine is running.
A calibrated spring closes the slide based on engine speed. The length and resistance of this spring are precisely calculated; a loss of spring tension will result in a mechanical failure. So, whatever you do, don't alter it.
Flat-type slides are more active than round-type slides and, consequently, they deliver superior performance. There's one negative aspect, however: friction is increased, which means the slide wears out more quickly. A stronger spring is therefore required to counter this problem. Incidentally, the throttle might feel stiffer as a result, which is why some manufacturers have added rollers for improved movement and reduced friction.
In order for the carburetor to operate perfectly, it must be totally airtight. The slightest hole, even the size of a needle, will lead to engine breakdown. The lifecycle of flat-type slides is limited compared to round-type slides. Also, they're much costlier to replace.
All twin-cylinder engines require the carburetors to be synchronized so as to get a clear engine response under acceleration. Static or dynamic adjustments are necessary. First, you have to make sure the throttle opening is perfectly calibrated by using a calibrated gauge rod. One of the two carburetors is considered as the master carburetor and serves as a reference. An idle screw allows you to open the throttle until the calibrated gauge rod goes through without any gap. It should slide easily, though (no friction). Once the master carburetor is set, go to the second carburetor. A screw in between the two will allow you to adjust the calibration of this carburetor. Again, open the throttle until the calibrated gauge rod goes through without any gap.
This method is recommended for preliminary adjustments, before you actually install the carburetors on the engine. Then, you have to perform dynamic adjustments, which require more precision and specialized tools.