® Mikuni vs. Keihin

Mikuni vs. Keihin
Many folks think "carbs are carbs." Yet even within a limited segment of the genre, carbureted powersports (motorcycles, to us) carburetors, there is considerable variation. For example, Mikuni and Keihin. Taken as a whole, Mikunis tend to be a little less sophisticated than Keihins. Castings are not as intricate, and circuits are designed to a different design ethic entirely. For one thing, Mikunis have fewer fuel passages than do Keihins, and those few passages are larger. Keihins passages by contrast are many and smaller. Mikuni's approach was always a large idle opening and make it up elsewhere to satisfy emissions requirements. Keihin went the other way and put the onus on the idle circuit and proudly made their jets non-removable and as tiny as possible. This accounts for the difficulty many have with Keihin idle circuits by the way, in terms of their being so ridiculously small as to be much harder to get clean. Probably the best example of this difference in design ethic is demonstrated when attempting to swap carbs from one engine to another. Mikunis are very forgiving. Their wide open circuit structure makes them easily adaptable. With Keihins, swapping can often lead to frustration.

Carburetor Circuits
Another place folks get tripped up when thinking that bike carbs are all alike is in blind endorsement of that ubiquitous, traditional 0-1/4 throttle (idle), 1/2-3/4 throttle (needle) and 3/4-full throttle (main) carb circuit function chart. Unfortunately there are a number of problems with it, not the least of which is it came from a particular class of carburetor. It was designed around a mechanical piston (technically, "variable venturi") carburetor, thus its application to CVs is greatly limited. CVs for example don't lift their slides in direct correlation to throttle opening, and in most cases don't even lift their slides until 1/4 throttle. Furthermore, since engine vacuum plays such a huge role in slide lift, such things as gear position, rpm, air temperature and a lot more go into when and how quickly the slide lifts. Thus a strictly per-slide or even per-throttle opening map of circuit function is completely inappropriate for a CV, a fact that both Mark Dobeck and Marc Salvisberg (Google them) quite famously put to use in establishing their respective businesses. Another fact that throws a monkey wrench into the use of published motorcycle carburetor circuit range charts is that many carburetor models have been made that "cheat," as it were, traditional thought on how circuits work. The Kawasaki KZ750 twin and Harley-Davidson Shovelhead models come to mind. The Harley carb has only two fuel circuits, main and idle, and the Kawasaki carb acts as if it does (though it has three) by its design. None of those traditional charts accomodate the somewhat non-traditional fact that these two bikes' idle circuits contribute some 50 percent of the fuel delivery (closer to 30 percent in the KZ, but still way more than is conventional -- the official manual has a nicely done fuel chart, check it out). And what do you do with the Yamaha 550 Vision carburetor, or for that matter Honda six cylinder Wing carbs that have electronic air bleeds? The game's up. No one-size-fits-all circuit map will work.

Carburetor Starting Circuits
Not all carbs even start their engines the same way. Some have chokes, some have enricheners, and some have ticklers, basically flooding devices. Richness at starting is of courae necessary because of the transition that fuel must go through from being a liquid to being a gas (that is, gasseous). It takes heat. When the engine is cold, its intake manifold is also, and very little of the wet fuel is converted, so more has to be supplied to compensate. A choke not only sounds different from an enrichener (though many manufacturers, thinking it would confuse, omit the name enrichener from their publications) it *is* different. A choke works by cutting off the engine's access to all but one circuit, the idle circuit. Engines using chokes, which means almost all Keihin carbs before about 1982, actually start on their idle circuits. Mikunis and later Keihins however uses the enrichnener system, which is so different as to need explanation. Four things define a carburetor: a fuel source, an air source, an air bleed, and an air control valve (note incidentally how many of these four criticial things are air related -- the carburetor is much more an air device than it is a fuel device). The enrichener is simply a second carburetor tacked onto (in some cases, such as the KZ750B and the Yamaha VMax, literally bolted to) the outside of the carburetor. Having all four of the same parts: fuel source, air source, air bleed, and air control valve, it's a miniature carburetor that is, like a siamese twin, joined to the main carb at only one point, the fuel source. They are used differently also. When using a choke, a little bit of throttle helps, because it adds the needle jet to the fuel exposed during starting. However, when starting an enrichener-equipped bike, use of the throttle will delay starting because with the carb and enrichener each having its own air source, opening the throttle actually diverts air away from the enrichener and to the carburetor, making starting more difficult.

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