The Constant Velocity Carburetor
During the late 1960s the third and final great leap in Honda carburetor evolution occurred. The solution was to combine the two carburetor types we explored in Parts 1 and 2, the fixed and variable venturi carburetors. The result was a carburetor with the fixed venturi's throttle butterfly and the variable venturi's slide--in essence two carburetors melded into one. As in the fixed venturi carburetor, the CV carb's butterfly throttle controls the airflow. And as in the variable venturi carburetor, the slide varies the venturi's size. And they work in synch. The fuel circuits work in the same way as in the variable venturi carburetor. How the slide is raised however is completely different.

How it works
Inside the carb's top is a rubber diaphragm. Attached to the bottom of this diaphragm is the slide. The diaphragm divides its chamber into two areas whuch will have different pressures. Below the diaphragm is atmospheric air pressure. Above the diaphragm is less pressure--venturi vacuum--routed there through a hole in the bottom of the slide. As the twistgrip is turned and the butterfly throttle is opened and engine rpm increase, airflow through the carburetor increases. This increased airflow results in the expected pressure drop in the venturi. This lower pressure reduces the pressure above the diaphragm, via the hole in the slide. However, the pressure below the diaphragm remains atmospheric. Thus a pressure differential between the two sides of the diaphragm is established. This differential is a force. It increases as the butterfly throttle is opened further, because airflow increases through the venturi, increasing the venturi effect and lowering the above-diaphragm pressure even more. At between one-quarter and one-half throttle, the force of this differential is high enough to overcome the weight of the slide and the force of its spring (if fitted). The slide therefore rises. More throttle opening results in the slide rising even further. Note that I said, "half throttle." Essentially, the constant velocity carburetor is a fixed venturi carb up to half throttle and a variable venturi carb after that.

Constant velocity
Where the constant velocity carb shines is in its achieving the carburetor's holy grail--the lack of pressure changes inside the venturi while the throttle is moving. As engine rpm go up and down, the CV's venturi velocity stays constant, making fuel discharge more consistent. More throttle opening increases the venturi's velocity, and the slide automatically rises and lowers this velocity back down to a baseline. Less throttle opening on the other hand decreases the carb's velocity, and the slide automatically falls to increase it up to the baseline pressure. The result is that intake velocity is constant despite continuous changes in throttle opening. The air pressure within the carburetor stays consistent. Thus fuel "pull" signals are immediately felt with every change in throttle opening.

High velocity
However, not only is the pressure differential consistent, it is also somewhat higher in the CV than in other carburetors. Like the variable venturi carb, the CV's venturi starts out very small but unlike other designs it stays that way until the throttle is nearly half open, at which point it begins lifting. This makes for high velocity. But even after the halfway point, the slide opens so slowly that its lifting doesn't linearly coincide with the throttle opening--the slide's opening is always slightly behind. Consequently, the carburetor maintains an extremely high venturi vacuum, resulting in instant discharge signals arriving at the fuel tubes at every change in throttle opening. The outcome is superior throttle response, so good that the constant velocity carburetor eventually became the best the OEMs had to offer. It even got smoothbore and flat slide technology added to it that made it the very best carburetor available.

Two CV slides
There are two types of slides used in constant velocity carburetors, the difference being how the negative pressure which operates the slide is contained. These are the rubber diaphragm and the metal labyrinth design. Honda’s earliest CVs were diaphragm type. This design had very quick response—too quick some would say when driveline snatch emphasized it, as was famous in the Honda 350 twin. However, this carburetor's real downside was the rapid deterioration of its rubber diaphragm. Ruptured CV carb diaphragms were common, and resulted in loss of engine function above half throttle. The metal labyrinth type CV came to dominate in the wake of the rubber diaphragm carb's disfavor. However, it too had a weakness. Its close-tolerance metal labyrinth seal was especially sensitive to dirt and contaminants, which plagued it all during the period of its use. In the 1980s the original rubber diaphragm CV reappeared in an improved form which all but elminated the earlier rubber diaphragm's problems. The new technology reestablished the CV carb's superiority and it was this improvement which carried the modern carburetor the rest of the way until fuel injection began to supersede powersports carburetors in the 1990s.

The air cutoff valve
About this same time, air cutoff valves also appeared to eliminate exhaust popping. When the throttle is closed suddenly from a large opening, the briefly still-present high engine rpm pulls on the only still-exposed fuel circuit, the idle tube. This strong vacuum overpowers the tiny idle passage, resulting an overly-lean mixture and thus poor combustion. As a result, a more than usual amount of unburned and partially burned contents enter the exhaust system. The heat of the exhaust combined with the constituents of previous exhaust gases and these residual unburned elements then results in combustion—inside the exhaust! That's the pop that you hear. Technically, this is called "afterburn", and you can see why. Operationally, the momentarily high vacuum triggers a diaphragm-operated valve that closes the idle circuit's air bleed, effectively super-richening the stuff going into the engine at closed throttle so it can be burned and does not then pass unburned into the exhaust. Thus no afterburn. No pop. However, understand something. The aircut valve is a consumer-confidence system, not an emsissions device. Moreover, defeating the valve is a good idea as it is redundant on properly adjusted carburetors and is otherwise a nuisance in that it has to be replaced at regular intervals due to deterioration.

It is noteworthy that Mark Dobek, the founder and original owner of the Dynojet company, made his fortune and reputation by understanding the constant velocity carburetor. At a time when everyone else was afraid of it, he figured it out. You might also be surprised to learn that Honda has used CV carbs in professional-level road racing.