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CV or VM Carbs?

  • Thread starter Thread starter hwb120240
  • Start date Start date
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hwb120240

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If I understand correctly the 1979 GS750L has VM type carbs.
What is the difference between VM and CV?

I have little experience with carburators and want to learn how to repair and what I'm repairing. My son has the GS and I am trying to assist with the restoration. I currently ride an '85 GV1200GLF which has CV carbs from Mikuni which I have had the "pleasure" of rebuilding. Is there much difference?

Don't mean to ask questions that have probably been aked many times before, but search did not help...

Thanks,
Wayne
 
CV carbs (80 and later) have a diaphram in the top. The diaphram is operated by the difference in the pressure acraost the carb, the diaphram moves a plugger up or down thereby increasing the size of the carb throat/bore (whatever wanna call it). Idea is to increase the size of the throat when airflow (cubic inches per second) increases to keep the velocity (inches per second) about the same.

Carbs previous to this did about the same but did it with mechanical linkage with the throttle to open similar plunger.

So as far as working on the carbs: the needles and jets and the floats and the float valves are perrty much the same.
 
Redman said:
CV carbs (80 and later) have a diaphram in the top. The diaphram is operated by the difference in the pressure acraost the carb, the diaphram moves a plugger up or down thereby increasing the size of the carb throat/bore (whatever wanna call it). Idea is to increase the size of the throat when airflow (cubic inches per second) increases to keep the velocity (inches per second) about the same.

Carbs previous to this did about the same but did it with mechanical linkage with the throttle to open similar plunger.

So as far as working on the carbs: the needles and jets and the floats and the float valves are perrty much the same.
Click to expand...

Cool! When I cleaned them originally I didn't see a lot of difference, but if I don't ask I'll wish I had,
 
Ditto everything Redman said.

The other major diff. is in the performance; CV carbs are much more responsive than the VM carbs. But something in me still prefers the VMs... nothing can go wrong with that direct linkage, whereas the frustration and expense of a holed diaphragm is not nice.

Cheers,
Mike.
 
VM "round slide" carbs are simpler. the throttle cables are connected to a linkage that raises & lowers the slides. The BS "CV" carbs have the throttle cable connected to linked throttle valves at the rear of the carb body. The slide is not directly linked to the throttle. It is controlled by vacuum on the downstream side of the venturi.
In order to use CV style carbs on your 79 750L, you'll nee an 80 or newer head, carbs, airbox & throttle assembly.
I hope this helped.

Terry
 
Last edited:
I'd argue the "CV carbs are more responsive than slide carbs" opinion, noting that all "performance" carb designs currently available from Mikuni or Keihin prefer slides (whether round or flat) to CV style carbs. CV carbs don't "flow" as efficiently as slide carbs either, so the 34mm CV's on a GS1000 only flow about as efficiently as a 30mm slide carb.

The benefits of a CV carb over a slide carb (and there are benefits) is that you generally have a much lighter throttle (only 1 cable, and you're not pulling against a heavy spring required to re-seat those slides) and better fuel economy. (the main reason that the manufacturers used them to comply with EPA emissions regulations)

The standard 34mm CV's that I took off a 1980 GS1000E and fitted on my 1981 GS1000ST are also more compact and much lighter than the "Australia only" 33mm slide carbs that were the original fitment on my bike. Cheers, Terry.
 
Hmm, thanks for your opinion there Terry. Sorry, maybe I haven't used the word "responsive" in the right way... I actually meant something more along the lines of 'snatchy' (although your views on that would be welcome also)... every bike I've ridden with CVs has always been more a challenge for low speed throttle control, vs. the VMs where I've found that things are much more civilised and controllable.

Interesting to hear the rationale that CVs are better on fuel, though. Thanks for the input. :wink:

Cheers,
Mike.
 
The VM carb can be just as responsive as cv carbs if it has accelerator pumps. The responsiveness of a carb is set by how accurately it maintains fuel mixture under dynamic conditions and not by maintaining a constant velocity . Dan
 
Thank you for all the input

Thank you for all the input

Folks I really appreciate the help.
Wayne
 
tfb said:
Hmm, thanks for your opinion there Terry. Sorry, maybe I haven't used the word "responsive" in the right way... I actually meant something more along the lines of 'snatchy' (although your views on that would be welcome also)... every bike I've ridden with CVs has always been more a challenge for low speed throttle control, vs. the VMs where I've found that things are much more civilised and controllable.

Interesting to hear the rationale that CVs are better on fuel, though. Thanks for the input. :wink:

Cheers,
Mike.
Click to expand...

G'Day Mike, no worries, and I agree that the CV's are "jerky", but this is due to the light springs required to return the butterflys, and not a result of their design. If you were to fit heavier return springs like the VM carbs use, the "responsiveness" would disappear.

I understand what you mean though, my BMW K1100LT is fuel injected and the throttle is lighter than that of our bikes fitted with CV's, and it's a nightmare to take your hand off the throttle to scratch your nose on a long trip! Cheers, Terry.
 
Good question, Wayne - Thanks for asking it!

Check out my post count ... I've been around "officially" for three years and unofficially (lurker) for several additional years, and somehow I never managed to learn either the differences OR which bikes had which type of carb.

This site rocks! You learn something new all the time, regardless of how long you've been here!

Regards,
 
Planecrazy said:
Good question, Wayne - Thanks for asking it!

Check out my post count ... I've been around "officially" for three years and unofficially (lurker) for several additional years, and somehow I never managed to learn either the differences OR which bikes had which type of carb.

This site rocks! You learn something new all the time, regardless of how long you've been here!

Regards,
Click to expand...

Have not been around here long but feel pretty comfortable amongst the inmates. :)
The two places I visit most are both motorcycle forums. Both have a pretty good bunch of folks to hang out with. I learn from both regularly, and not strictly about motorcycles.

I asked about the carbs because the first set I ever worked on are the ones on my Madura. They are CV. I could see a difference but not "understand" the diff. Like most I learn best from hands-on, so I like to ask before I @#$% it up, LOL. That way maybe I can tell if I'm accomplishing anything. Other than disassembly I mean.

Wayne
 
Another big difference, which I'm surprised hasn't been brought up, is with synchronization. CV's are "simple" since the linkages are all exposed outside of the carb. The VMs require that the top of the carbs be removed in order to reach the set screws. This may be different for certain VM equipped models, but I ran across it on a 550.

Brad bt
 
terry said:
G'Day Mike, no worries, and I agree that the CV's are "jerky", but this is due to the light springs required to return the butterflys, and not a result of their design. If you were to fit heavier return springs like the VM carbs use, the "responsiveness" would disappear.
Click to expand...

Very interesting.

Has anyone done this?
 
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