® CV carburetor vacuum tops

I think this one was put to bed a long time ago, but I still hear it every once in a while. The notion that Honda (actually, Keihin) made the Keihin constant velocity carburetor tops and slides as a matching set. No, not so. Good thing, too.

First, the clearance between the top's brass bush and the slide's chrome plated shaft is not so meticulous that such pains as special fitting needed to be taken. We're talking five or more thousandths of an inch here, a specification that in the world of Japanese engines is quite large. Sloppy, even. Second, the slide's vacuum seal is itself very basic. This is not a high precision part. Labyrinth seals are fairly low grade seals, relying as they do on trapped air and not physical contact. Third, there has been over 40 years of folks mixing and matching, to coin a phrase, tops and slides with never an issue. Some have even used CB450 and GL1000 tops interchangeably, and these actually are slightly different designs, yet they still work very well.

Well then. What's up? It's possible this mistaken theory of matched parts started when someone noticed that the slide was not available from Honda without also purchasing the top. They were sold together as a set. But that didn't mean they were matched, only that they were sold together. And they were sold together for business reasons having nothing to do with their specification. Career mechanics have long observed this. It comes about when the motorcycle manufacturer's suppliers make deals on which parts will be available as spares, in what quantity, and in what kind of muliples or configurations. Hence carb drain screw o-rings that came only in "packing sets", jet needles and needle jets that were sold together, and float valves and their seats available only paired. That sort of thing. Not matched parts. One more example of folks having little or no background in the industry jumping to unwarranted conclusions.


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