The Variable Venturi Carburetor
The first-generation carburetor described in Part 1 was pretty crude. Due to its wide open venturi, vigorous throttle opening would make it go way lean--and sudden closing, way rich. And engineering-in creative jetting couldn't fix it. Even making the carb's venturi very small--which guaranteed a more consistent pull on the fuel tubes regardless of throttle position--was tried to good effect. However, that restricted the engine's maximum airflow just when the going got serious, which limited rpm and power. The solution was slow in coming because the car industry pretty much controlled the technology back then and their application of this carburetor was not as glitchy as in motorcycles because their carburetor fed several intake ports at the same time, which by itself steadied the intake vacuum. The ultimate answer to the fixed venturi carburetor's inconsistent delivery in motorcycle application came in the form of a venturi which varied in size. It could be small enough to maintain good velocity and then grow incrementally while still maintaining velocity, when more airflow was needed for maximum power. By sticking a sliding gate inside the venturi, the venturi could be carefully matched in size for each airflow volume, minimizing changes in negative pressure in the venturi. This "variable venturi" carburetor maintained a high velocity at both large and small throttle openings, and was a simple and elegant solution. It won instant approval and quickly became the carburetor of choice for almost every motorcycle ever made and the design which even today virtually defines the carburetor in the minds of most enthusiasts.

The needle jet
So much higher was the variable venturi carburetor's average venturi velocity that it needed a tapered valve inserted into the main tube to prevent it from discharging all the time. This tapered valve is called a "jet needle", and its importance can be intuited when one considers that it controls the bulk (1/4 to 3/4 throttle) of the variable venturi carburetor's operating range. The jet needle is attached to and moves up and down with the throttle slide. The needle fits loosely into the "needle jet", the new name for the main tube. The needle's taper results in more fuel flowing out of the needle jet as the slide and needle are raised. And it is adjustable. Adjusting it upward in the slide exposes more of the needle jet at a given throttle height, resulting in a richer air/fuel mixture. Repositioning it lower on the slide exposes less of the needle jet at that same throttle height, resulting in a leaner air/fuel mixture. You need to know three things: One, the needle jet and jet needle are the fastest-wearing carburetor parts. Two, they are the parts discontinued (made unavailable) by Honda for for almost all 50-year-old motorcycles. And three, the needle jet and jet needle are the parts most needful of changing to alternative types when significant changes are made either to the engine or to the carburetor. An unfortunate perfect storm, that. The needle jet and jet needle make up the most challenging part of owning a vintage Honda today.

Air bleeds
As mentioned in Part 1, all carburetor circuits have air bleeds that make the fuel rising in them bubble so that when discharged the fuel is in droplet form, which the engine's vacuum converts to a mist, and its heat converts into a burnable vapor. The air bleeds on most variable venturi carburetors are found on the back side near the air filter because that is what the air bleed supplies to its fuel circuit--filtered atmospheric air. In virtually all production streetbike carburetors the air bleeds are pressed-in and not removable. But they still have to be cleaned just as the fuel jets and circuits need to be.

The keyhole carburetor
Although the variable venturi carburetor did much to guard the important venturi negative pressure, there was still air residing beneath the slide that would be compressed and uncompressed each time the slide went up or down, resulting in slight delays in discharge signal as the air took time to recover its median negative pressure. Keihin/Honda addressed this by boring the venturi with two radii stacked vertically to create an oval venturi. In this way, the area under the slide was narrowed just as if the venturi was smaller, but since the vertical oval made up for the lost area, the venturi's size wasn't actually reduced. The carb "thought" it was smaller than it was. Honda appllied this oval or "keyhole" venturi design to many of its variable venturi carburetors in the 1960s and 1970s. Eventually a better solution to managing under-slide negative pressure would be introduced in the flat-slide carburetor. More on that one later.

Bypass ports
The first carburetors were pretty crude. With basically two circuits: idle and main, if it wasn't for the gross overlap of the main circuit, there wouldn't have been any midrange at all. In fact, blubbery richness of the main circuit meant this circuit extended almost down to idle. But as carbs got more sophisticated their circuits shrank in size and multiplied in number--they became more like sprinklers and less like fire hoses. Circuits were tightened up, made maller, more precise, more efficient. And the idle circuit had to fend for itself. Out of these changes came the "bypass port". The modern carburetor's idle circuit is pretty complex--it has as many as eight openings. One of these is a tiny opening (or "port") ahead of the throttle, called the "idle discharge port". This port is always exposed to the engine because the engine is expected to idle with the throttle virtually closed. Slowly open the throttle however and you'll uncover two to three additional ports, similar in size to the idle port and very close to it, and usually staggered in their placement so as the throttle is opened it gradually exposes more of them. These auxiliary ports supplement the idle port and fill the fuel delivery gap between the idle and the next circuit--the needle jet--providing transition between the two.

Beware
Because the bypass ports are fed by the idle jet, they are part of the idle circuit. But unlike the idle port, the bypass ports are not controlled by the idle mixture (pilot) screw. This means that if for some reason a too-large idle jet is installed in the carburetor, the problem will be the non-tunable bypass ports. While the pilot screw can adjust out the resulting excessive discharge at the idle port, it can't do anything about an overly-rich bypass port discharge. This is just one of a number of reasons increasing your carburetor's idle jet size is fraught with potential misgivings.