® Carburetor Evolution

Carburetors, like most technologies, definately have undergone an interesting progression.

Fixed Venturi
Carburetors after the teens and into the 1960s (in a few cases well into the 1980s) were mostly a type known as fixed venturi. A fixed venturi carburetor is one that is basically a casting with a big hole in it, and the hole is opened and closed with a flapper valve, i.e. throttle plate (butterfly). Same as most car carburetors, not to mention lawnmowers. This kind of carburetor did a reasonable job when engine rpm ranges were still fairly narrow, but its fixed opening size meant that each time the throttle plate moved, the negative pressure signal that coaxed fuel up its tube also changed. Opening the throttle raised this pressure, closing it lowered it, just the opposite of what the carburetor needed. The result was sloppy carburetion and hesitant engine performance, but this was taken for granted. No one expected more. Moreover, the drawbacks were somewhat mitigated on engines where more than one cylinder drew from the same carburetor, making intake pull stronger and more regular. Then the fixed venturi carb was okay. But just okay, and on motocycles, not so much.

Variable Venturi
Then someone removed the throttle butterfly and replaced it with a valve that slid vertically at the center of the venturi, making the slide both a throttle valve and a venturi resizer at the same time. Since the venturi was resized each time the throttle moved (it couldn't help but do so, as the throttle now *was* the venturi), the change in venturi signal that plagued the fixed venturi carb was largely compensted for by the venturi resizing itself to match, thus stabilizing the fuel signal. A neat and simple design.

Constant Velocity
As good as the variable venturi carb was, and it was good enough to completely change the game for motorcycles, immediately and for a very long time becoming the epitome of performance carburetion, riders' expectations also became more demanding. A further leap in consistent mixture delivery was sought. Strangely, the solution was to add the fixed venturi carb's throttle plate back in, combining it with the variable venturi carb's slide valve, to create a new carb having both butterfly and slide, that would be called the constant velocity (CV) carburetor. The CV carb gets its name from the fact that it was the ultimate solution for this quest for consistent mixture delivery despite changes in throttle opening while riding. See my article on How CV Carbs Work. In short, each change in throttle plate opening is accompanied by an ideal slide height, the correlation controlled by the amount of negative pressure relayed by the throttle and felt by the slide's vacuum diaphragm. An ingenous solution and one that results in the most constant over-the-jet signal of any carburetor design to date. No gasps, no hesitations, just seamless fuel delivery.

Last updated January 2022
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