High performance
It was inevitable that in the process of all this development, some very important innovations would emerge. The following are two ways carburetors were refined to perform better. Though originally viewed as high performance or racing specific improvements, these are now almost taken for granted today.

The smoothbore carburetor
The "smoothbore" carburetor is the carburetor equivalent of a wide range spark plug. It's a carburetor whose effective rpm range is broadened. A carburetor body is bored once to create the venturi and a second time perpendicular to that to form the vertical tunnel that houses the slide. Where these two intersect are found airflow-disrupting machined edges. A smoothbore carburetor contains an aluminum or plastic insert that covers these edges to reduce air friction inside the venturi. The big deal here is that this carburetor flows more air at all throttle openings, and thus offers the performance of a larger carburetor. But since it isn't actually larger, it retains good low speed performance at the same time that it increases high speed performance. Its advantage isn't in the texture or "smoothness" of its bore. "Smooth" is not really an accurate term here. "Straight" is closer. "Undisturbed" even better. It's merely the absence of sharp edges in the bore. Polishing the interior of a carburetor doesn't make it a smoothbore carburetor. The smoothbore carb is a pretty cool deal, and a real-world advantage. All of today's high performance carburetors are smoothbore carburetors.

The flatslide carburetor
The "flatslide" carburetor was developed to improve throttle response. The throttle slide in the earliest variable venturi carbs was round. This made the air space beneath it large in area. When a slide is raised and lowered with the twistgrip, this under-slide area changes in volume, and the negative pressure that is always there for a split second has to restabilize. This happens each time the throttle is used, resulting in slightly interrupted fuel discharge. We've discussed the venturi's inherent air pressure inconsistency issue already and how it drove carburetor innovation. In a variable venturi or CV carb in street use, the inconsistency brought by the underslide area is not felt by very many riders. But in racing, it's a big deal. Consequently, a slide was developed that was rectangular in cross-section instead of round. It's more gate than valve. Now when the throttle is jerked open, the area that has to normalize is much smaller, thus the pressure adjusts itself faster, with the result that the carburetor's response is more instantaneous. This "flatslide" carburetor therefore is the most highly developed version performance-wise of the variable venturi carburetor. It is even found in many OEM applications in addition to its being considered the best high performance aftermarket carburetor. There is a drawback however. Those flat surfaces add more friction and thus the slide tends to stick. This is why OEM versions are slightly different from the ideal racing design, incorporating a slide that is flat on the air filter side and round on the engine side, and why the most sophisticated racing flatslide carbs have roller bearings in their slides.

Part 5