Wing tips #12

[mikenixon]

This tip is about wing carb oddities and misconceptions.

Vaccum tops and slides
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 GL1000 carburetor's tops and slides as a matching set. Or even the GL1100's. No, not so. Good thing, too. Consider the following. 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, 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.

No overflow system
The Gold Wing stands out among Keihin carbs in one intriguing peculiarity--none of the carbureted Wings has an overflow system. By an overflow system I mean a way for fuel above normal design level to become evident externally. To be seen. Most Keihins have such a system. It's simply a brass standpipe inside the float bowl that diverts outside fuel that somehow reaches excessive height. Just as the standpipe in a toilet diverts excessively high tank water straight into the plumbing. Granted, there are some Keihins on a number of later Hondas (such as all V4s and most in the CBR family), and even some Kawasakis, that also do not have overflow systems, and also a few more examples of Mikuni-equipped Yamahas, etc. without this important feature. But in the overall powersports carburetor world they are relatively very few. And I find it especially strange that the Wing, through all the years it was carbureted, never had the system the riders of most other machines take for granted, which visually warns of overflow. This is odd. For, on any bike without an overflow system, including the aforementioned Gold Wings, when the carbs overflow--and this is not such an unusual event--there is no telltale, no visual clue. Two things result from this. One, it means overflow can happen, whether from dirt around the float valve, a blocked bowl vent, or an issue with the float itself, and you never know it. There is no sign. Not good, I think. Because the second thing is, this fuel is going somewhere and not onto the ground where it is visible and can serve as a warning, but into the engine. The fuel enters the engine and eventually makes its way into the crankcase, where it dilutes the engine oil. On the Wing, the fuel often also backs up inside the air plenum, puddling there and then when dry new layers puddling over it again, and those drying also, eventually forming a surprisingly thick layer of hard, almost epoxy-hard varnish resembling tree sap, that takes more than a whole day in the ultrasonic to dissolve. Been there, done that. As I mentioned in another article, this is what causes throttle shafts to become twisted also.

Car carburetor tech
Ever notice that small, stubby brass pipe sticking out on the air plenum side of the individual carburetor body? That's the float bowl vent. Yes, the float bowl vents through the air filter. Here's another one. Everyone is familiar with the short rubber hoses that connect each carb body to the air plenum. What are they for? They're not fuel passages. And though they do carry air, guess what, it's not low pressure; it's not vacuum. It's atmospheric air (remove those hoses and absolutely no performance change will result). But atmospheric air to where? To the idle circuit. So each carb's idle circuit get its bleed air through the air filter too, just as the bowls do their vent air. Both are given atmospheric air that is filtered. Although these examples of convoluted carburetor air venting would become common on motorcycles in the early 1990s, it was rare on bikes in 1975. But not so rare on cars. Commonplace, in fact, due to emissions becoming a concern on cars some twenty years before it was on the bikes. And where did the first-generation Gold Wing carburetor originate? Yup, on a very early Honda car, the N600.

[Rat]

Great stuff as always !

Gord

[mikenixon]

Thanks Gord!

[Whiskerfish]

Good read!!

[sunnbobb]

Another wing lesson, thanks

[77Gowing]

Mike, that was very good. I'm quite ignorant when it comes to carbs, but you write very clear and informatively. I sent my set out. This is one area where I let someone else do the wrenching. As Eastwood says, "Man's gotta know his limitations!"

[mikenixon]