The smoothbore carburetor
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 flow-disturbing 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. It's advantage isn't in the texture or "smoothness" of it's bore. It's in the absence of sharp angles in the bore. A pretty cool deal, and a real-world advantage.

The flatslide carburetor
The throttle slide in most variable venturi carbs is round. This makes the space beneath it large in area. When the slide is raised and lowered with the twistgrip, this under-slide area changes in volume, so that 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. In street use this 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. 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. The "flat slide" 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. The is a drawback however, and that is the flat slide carburetor's slide having higher than average surface friction and thus tending to stick. This is why OEM versions are slightly different, 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.

Rebuilding carburetors
Carburetors have air bleeds which feed atmospheric air into the fuel circuits to make the fuel pre-atomize as it rises into the venturi. Thus every atomizing circuit in a Honda carburetor describes a "Y" shape, with solid fuel at the bottom of the "Y", air on a second branch, and fuel and air together on a third branch. All three legs are critical when cleaning carburetors, but many overlook this. Ultrasonic is now the preferred cleaning method and that is a good thing considering the mess and hazards of traditional cleaning chemicals. Ultrasonic works well but only when the ultrasonic machine is high enough power. Ultrasonic cleaning does not replace hand vetting and cleaning however. That still needs to be done. Each circuit, each passage, needs to be cleaned with cleaner aerosol and confirmed visually. In modern times rebuilding 50-year old Honda carburetors has reached a crisis. Honda has run out of the most critical parts, the needle jet and jet needle and some cases even the float valve. This is not a good thing, as aftermarket substitutes have proven of very poor quality. Despite the opinions of some, wet blasting is not a substitute for carburetor cleaning. Blasting is not cleaning. It is a carb body exterior refinishing process, not an internal passage resin removal process. And about the throttle shaft seals, there are no engine performance issues that can be solved by the replacement of those seals. They do not seal vacuum, they ward off dust. Go after the things that really do hurt engine efficiency. Cylinder compression, valve adjustment and ignition set up and timing are at the top of the list.

Carburetor rebuild kits
Honda never sold rebuild kits for its production powersports carburetors. Instead, the company has traditionally supplied "packing sets". "Packing" is another word for gasket, thus in reality these were carburetor gasket sets. They included some sealing o-rings, but no diaphragms, float valves, metering parts or screws. Which is normal. You could buy all of these things individually from Honda, just not as part of the packing set. One of the most perverse things in this industry is how inundated it has become with carburetor aftermarket rebuild kits. These contain metering parts in addition to the gaskets. This is unfortunate. The most important parts of the carburetor, the parts with the most engineering and the most effect on the bulk of the carburetor's function, you're going to replace with cheaply-made junk? I liken carburetor rebuild kits to Chicken McNuggets. The popularity and success of both derives more from marketing than from substance. They are hollow products. Even the kits carrying the most respected names ruin carburetors. Their jet needles and needle jets in some cases are so far off the factory dimensions that they permanently make the carburetors work badly. What do you think the chances are that a company who claims their part fits two or three different models will properly substitute for a part the factory has exerted its considerable resources to design to the extent that it puts a different jet needle into each year of a given model? I wouldn’t take that bet.