Originally Posted by creeper
You asked for it Shawn...
-Without going into excruciatingly painful and boring detail, there are three types of carburetors in common use. There is the fixed venturi, the variable venturi and the constant velocity.
-The fixed venturi is just as it sounds… a venturi with a disc or butterfly valve on a rotating shaft. Once popular on Harleys and such, it isn’t used much anymore. The most common to be found is the aftermarket S&S carb series for Harleys.
-The next, the variable venturi is represented by the current crop of high-performance and competition 4-stroke carburetors like the Mikuni HSR and TMR series and the Keihin FCR series.
-The “size” of the venturi is established by the position of the slide, which in turn is controlled buy the operator’s skill level. The negative pressure or vacuum in the venturi area, and the volume of fuel drawn into it varies with the slide position, engine speed, load and elevation.
-When used by an operator who understands the design, how to tune them correctly… and has the skill to benefit, a variable venturi carburetor offers near instantaneous throttle response.
-The first two carbs require an accelerator pump to offset the loss in negative pressure or vacuum that occurs when the throttle is opened abruptly or at too low an RPM to maintain that vacuum.
-The air in the column responds virtually immediately, but the fuel, having temporarily lost its high vacuum signal, takes a moment to catch up to the shift in pressure vs. area.
-The last, the constant velocity carburetor is, in very simple terms, a combination of a fixed and a variable venturi… to a point.
-The operator has complete control over the butterfly valve downstream of the vacuum piston or slide but… the piston itself is controlled not by the operator, but by the balance of pressure in the carburetor.
-Negative pressure or vacuum, relayed via transfer ports in the bottom of the piston to the area above the vacuum piston’s operating diaphragm, and positive pressure beneath the diaphragm relayed via an atmospheric port, are what control the pistons movement and position.
-An accelerator pump is not needed in a CV carb because in theory, velocity thru the venturi is held at a constant and high negative pressure. If there is no abrupt loss of that high vacuum signal, then fuel delivery to the venturi is also constant and determined only by jet and needle sizes.
-The operator can send a pressure “signal” by closing or opening the throttle, but if the signal is inappropriate for the manifold pressure at the time, the vacuum piston serves to “dampen” the engines response to that signal while maintaining that all important high vacuum signal.
-To put it another way, if the operator is unskilled at matching RPM, load and throttle position, the constant velocity carb will dampen his or her mistakes… to a degree.
-The CV carb is actually more efficient than a variable venturi carb in the sense that it uses less fuel to produce a response, but due to the damping characteristics, that response may not be as quick to occur as some would like. In a racing application, the delay, no matter how short, is unacceptable.
-There's lots of other stuff… but this should give you a pretty good picture of the differences.