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CV carbs, vacuum slide VS cable slide?

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23K views 7 replies 3 participants last post by  sweatmachine  
#1 ·
I've noticed that alot of smaller bikes use a cable lifted slide while the larger ones use vacuum lifted slides like the Harley CV's. What are the advantages and disadvantages to either style, and is there a hard line where you'd want to use one or the other? Just curious.
 
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#2 ·
Bugman said:
I've noticed that alot of smaller bikes use a cable lifted slide while the larger ones use vacuum lifted slides like the Harley CV's. What are the advantages and disadvantages to either style, and is there a hard line where you'd want to use one or the other? Just curious.
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Only one of those carbs is actually a CV carb... the one with the vacuum operated slide. The mechanical slide will open quicker but it will not automatically adjust for changes in altitude, air density (temp changes), etc. the way a CV carb does.

-Craig
 
#3 ·
caschnd1 said:
Only one of those carbs is actually a CV carb... the one with the vacuum operated slide. The mechanical slide will open quicker but it will not automatically adjust for changes in altitude, air density (temp changes), etc. the way a CV carb does.

-Craig
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that's a little misleading, a CV doesn't actually "adjust" for anything. The vacuum-operated nature of a CV carb will respond better to those changes but I think the difference is negligible.

Mechanical slides, like roundslides and especially flatslides are better performers than CV carbs for a few reasons. As already mentioned they open quicker which equals immediate throttle response. The only problem with that is when you whack open a huge carb the vacuum drops at the venturi and the engine goes lean due to not being able to suck in enough fuel. Injector pumps solved that problem and it's pretty much a non-issue on pumper carbs.

Also, with a flatslide at full throttle the slide goes up totally out of the intake path so you get the full bore unobstructed vs a CV carb will always have the butterfly in the middle of the throat. In other words, a flatslide of the same diameter will always flow more than a CV carb.

I have a pair of Mikuni HSR 42mm flatslides on my bonnie. They have accel pumps and run AWESOME! They replaced the 36mm POS CV carbs that came stock on the bike and I picked up a ton of power.

No matter what bike (big HD, small Brit bike, small Jap bike, etc), I'd rather have flatslides (or even roundslides) than CV carbs anyday.
 
#4 ·
The difference is not negligible, in fact it's quite significant. Because the CV adapts to the air density it keeps the air fuel mixture constant at different temperatures and altitudes. You can take a CV from sea level to 12K feet and not make a single adjustment. A mechanically controlled slide cannot do this. You'll either be way too lean as sea level and run better at altitude or you'll run fine at sea level and foul your plugs at altitude. The mechanical slide is a better performing butterfly (because it opens up the throat entirely).

It's true that the mechanical slide performs better then the CV. The CV is best suited for someone who isn't trying to eek out every last bit of power from their bike but wants the best drivability. Because of it's flexibility across differing temp and alititude ranges it is a more popular carb for a bike that will be used to cover large areas.

If I was racing, I choose the mechanical slide and jet it on race day for the conditions. If I have a bike that I want to ride everyday and not have to mess with the jetting for summer, winter, the day in the mountains, etc., I'll choose the CV.

-Craig
 
#6 ·
caschnd1 said:
The difference is not negligible, in fact it's quite significant. Because the CV adapts to the air density it keeps the air fuel mixture constant at different temperatures and altitudes. You can take a CV from sea level to 12K feet and not make a single adjustment.
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that's not entirely true. A CV at high altitude will need leaner jetting than a cv carb at sea level. Example: I rode my Bonnie with the stock CV carbs from Phoenix to Flagstaff a couple of years ago. It was running perfect in Phoenix (at/near sea level) and farting rich at Flagstaff up in the mountains. A CV carb might respond better to these conditions than a slide carb but it doesn't keep the mixture constant in all conditions. No carb can do that.
 
#7 ·
sweatmachine said:
that's not entirely true. A CV at high altitude will need leaner jetting than a cv carb at sea level. Example: I rode my Bonnie with the stock CV carbs from Phoenix to Flagstaff a couple of years ago. It was running perfect in Phoenix (at/near sea level) and farting rich at Flagstaff up in the mountains. A CV carb might respond better to these conditions than a slide carb but it doesn't keep the mixture constant in all conditions. No carb can do that.
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Can't argue with your experience. Only thing I can say is mine is totally different. I run from Phoenix to Flag regularly and never have a problem. Flag is only 4,000 feet higher then Phoenix. Not much of a difference really. I've run as high as 10,000 ft on my bike (9,000 feet higher then Phoenix) and never swapped jets or experienced any drivability problems. Also, CV carb farts are typically a sign of a lean condition, not rich.

All that aside, we are talking about different CV carbs entirely. All my CV experience is on the Keihin CV40 (Harley) carb. I don't have experience with other CVs that may not be as "user friendly".

-Craig
 
#8 ·
caschnd1 said:
Can't argue with your experience. Only thing I can say is mine is totally different. I run from Phoenix to Flag regularly and never have a problem. Flag is only 4,000 feet higher then Phoenix. Not much of a difference really. I've run as high as 10,000 ft on my bike (9,000 feet higher then Phoenix) and never swapped jets or experienced any drivability problems. Also, CV carb farts are typically a sign of a lean condition, not rich.

All that aside, we are talking about different CV carbs entirely. All my CV experience is on the Keihin CV40 (Harley) carb. I don't have experience with other CVs that may not be as "user friendly".

-Craig
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well I didn't mean farting rich literally, probably a bad choice of words on my part.

The CV carbs that were on my bike were also Keihins but cv36s, pretty much the same as yours but smaller. Now I have a pair of HSR 42s so I don't have to mess with those CVs anymore :D
 
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