MV and CV and... huh?

[Clumzi]

Alright, I've been poking about in the carb specs sticky page at the top and I'm curious about something. Are the specs for all bikes with the same model carbs (ie 77-79 that used the CV carbs and the 80-whenever that used the MV carbs) the same regardless of displacement? I was looking at the carb specs for a 1981 850 and they look exactly like the specs I found for a 1980 550. Are there any differences?

[skidMarkNZ]

Carb model only denotes the main housing itself, apart from that i would say only jets were changed.

[Billy Ricks]

What Mark said. Of course some bikes use different size carbs as well. And you have it backwards, the VM series carbs went on the older bikes, CV on the later bikes.

[alke46]

Please indulge me while I am not trying to hijack this thread, can someone tell me what the vm and cv stands for? I thought the cv stood for constant velocity, yes or no? If that is the case, then what does the vm stand for? Thanks

[76FJ55]

you are correct the CV stands for constant velocity. the CV carb models on the early 80s GS were BS models. The VM model carbs are actualy slide valve carbs. the VMs happen to have round slides. another example of slide valves are RS carbs they however have flat slides.

the CV carbs all termed that due to the vacuum opperated slide which maintains are relatively constant air velocity through the throatof the carb. and use a butterfly valve to control the amount of air which flows through the carb.

In slide valve carbs the slide controls the amount of air passing through the carb. the slide is connected dirrectly to the throttle cable so when the slide position changes the velocity through the carb changes relative to throttle positiona and engine RPM.

[Steve]

Please indulge me while I am not trying to hijack this thread, can someone tell me what the vm and cv stands for? I thought the cv stood for constant velocity, yes or no? If that is the case, then what does the vm stand for? Thanks

You have to understand that we are all mixing up terminology and letters here.

VM and BS are model names (numbers?) for the Mikuni carbs that were used.

CV is a type of carb, and you are right it is constant velocity (some call it 'constant vacuum'). There are many different carbs from the different manufacturers (Mikuni, Kehin, Bendix, SU, etc.) that are constant vacuum carbs. Most of our GS bikes that use CV-type carbs happen to use Mikuni BS series carbs. Most of the earlier GS bikes that use slide-type carbs use the Mikuni VM series.

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[alke46]

Thanks Steve, that pretty well clears it up for me.
I've not had much experience with the cv carbs but I sure do like the simplicity of the vm sliders.

[Steve]

Actually the CV carbs (the BS series) are not any more difficult. In fact, I prefer them to the VMs.

With the VM carbs, you have adjustable pilot fuel screws (under the carbs) and adjustable pilot air screws (on the sides of the carbs) and you need to adjust both of them to get the right mixture and the right amount of that mixture.

With the CV carbs, the mixture is pre-set with the pilot fuel jet (in the float bowl, next to the main jet) and the pilot air jet (in the carb's intake throat). The 'idle mixture adjust screw' that is on top of the outlet of the carb only controls how much of that pre-set mixture gets to the engine.

There will be differences for those who are trying to tune their carbs for big-bore kits, cams and other goodies, but either series carb can be tuned for them, you just have to change different parts.

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[alke46]

The worst adjustment I see is trying to adjust the fuel screw underneath with the carbs mounted on the bike. I am trying to do that right now and am ready to pull the carbs off just so I can adjust the two inside units, # 2 and #3. What buggers!!!

[Steve]

I had to look back through some of your previous posts on other threads to see what bike is mentioned most often. Apparently you are working on a '79 1000?
(This is why it's nice to put it in your sig line, so you don't have to add it every time and we don't have to go digging.)

Motion Pro offers a tool that makes this easier without pulling the carbs, but it's $30.

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