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CV carbs vs other carbs...

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Guest

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Hello all. In my dealings with the carbs on my '81 GS850G, I have learned that it has CV carbs - and that they are apparently fundamentally different from "other" carbs. I'm pretty sure that "CV" stands for Constant Velocity, but I don't really understand what makes them different. Is there a thread or a tutorial anywhere on TGSR that explains them? I'd really like to know what I have here.....

Scott
 
CVs arent mechanically linked like VMs are. My 81 750 has CVs and I find them rather twitchy as compared to the VMs when appliing vry minute throttle grip twist. CVs seem kinda grabby to me.
 
The whole concept of "constant velocity" is based on a "variable venturi", which is provided by the slides. At low throttle openings, there would not be enough velocity past the jets to do a proper job of atomizing the fuel. Lowering the slides reduces the opening, which increases the velocity. In a venturi, there is an inherent pressure drop that goes with the velocity increase. The hole in the bottom of the slide goes to the chamber above the diaphragm. When the air flow increases, so does the pressure drop in the venturi. That lowers the pressure above the diaphragm to the point that the slides are lifted, allowing more air flow. The change in venturi size keeps a relatively constant velocity across the jets, making for better fuel mixing. This also has an added advantage for altitude compensation, as the jetting requirements don't change much, if any, as you go up in altitude. Throttle resonse remains crisp coming off-idle, even though the carbs seem to be way oversized, compared to the VM series slide carbs.

One disadvantage that I can see with CV-type carbs is in racing. Any time you close the throttle to shift, air flow through the carbs will drop, likely to the point that the slides will drop. When you open the throttle after the shift, it will take some time to get the slides back up. Might be just a small fraction of a second, but do that a few dozen times per lap, you could easily lose a few seconds per lap. On the street, that is not as much of a concern, and the better driveability easily offsets that, so my choice would be slide (mechanical) carbs for the track and CV carbs for the street.

.
 
IN a nutshell, CVs compensate for elevation changes, and helped when getting carbureted bikes through the EPA was a problem. The Other carbs do no elevation compensation at all, but they are easier to tune and have much quicker throttle response. Most riders prefer them, except the guys like me who ride up and down huge elevation changes every day. I have all of my bikes switched over to CVs, they run great from sea level to 14,000'.
 
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My Suzukis with CV carbs have a significantly easier throttle pull than my CV equipped bikes. I like that, and I also like the tendency for CVs to correct for elevation changes.

All my riding life, I've been predisposed not to like CVs. I was used to Amal and Mikuni slide carbs, and the extra complication and unfamiliarity made me not like them. In practice, on a touring bike, I now prefer them. Of course, because my touring bikes use them, I now am predisposed to want stock exhaust and intake on my old touring bikes.
 
Thanks to everyone. I knew I could count on y'all to set me straight. At the suggestion of Tom203, I did Google CV carburetor theory and found a ton of returns, but this Youtube video makes it really clear. After the reading I've been doing in the carb rebuilding tutorials, I was familiar with all the terminology in this video, which clarified them all. Here's a link if y'all are interested...

And Steve - so the term Constant Velocity was adopted because the design maintains a (relatively) constant velocity of air past the jets. I learned something today; how cool is that?

https://www.youtube.com/watch?v=wyspAHrMbb8
 
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