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CV & VM Carbs

  • Thread starter Thread starter markwh
  • Start date Start date
M

markwh

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What's the difference between CV carbs & VM Carbs? They're both vacuum carbs, right? Does a 1978 GS1000 has VM carbs?
 
VM carbs have mechanical slides - twist the throttle and the slide lifts.

BS/CV carb slides lift due to vacuum - twist the throttle and a butterfly valve opens which allows the engine to draw more air thus lifting the slides.

All GS bikes larger than 450 used mechanical slide carbs through the 1979 model year. In 1980 they switched to CV type.
 
markwh said:
What's the difference between CV carbs & VM Carbs? They're both vacuum carbs, right? Does a 1978 GS1000 has VM carbs?
Click to expand...
First of all,
welcome1.gif


BassCliff will be along shortly with the 'official' welcome, but I will try to answer your questions in the meantime.

The difference between VM and CV carbs? First of all VM is a model name, CV is a type of carb. I have heard it as "constant vacuum" or "constant velocity". The carbs that Suzuki chose to use on the GS series are Mikuni BS carbs, which replaced the Mikuni VM series.

The biggest differences between the two are that the VM series carbs have throttle slides that are lifted directly by moving the throttle grip. The BS series open butterfly valves when the grip is twisted. The slides that are just upstream of the butterflies are lifted by vacuum when engine speed rises enough to create a vacuum behind the partially-closed slides. These carbs were more EPA-friendly in the time when emissions were coming under scrutiny because they eliminated the 'bogging down' or 'flat spot' when the throttle was suddenly opened.

Your 1978 GS1000 will have the earlier VM carbs. The easiest way to tell one set from another is by looking at the tops. The VMs have a rectangular, domed top that is held on with three screws. The BS carbs have a larger, mostly round, flat top that is held on with four screws.

Here is a parts diagram depiction of the VM carbs:
VMcarbs-1.jpg


Here is a parts diagram depction of the BS carbs:
BScarbs.jpg


.
 
Last edited:
Greetings and Salutations!

Greetings and Salutations!

Hi Mr. Markwh,

You'll find carb manuals for both the VM and BS(CV) Mikuni carbs on my website, as well as copies of the clean-up series for both. Let me just roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike! :D

Thanks for joining us. Keep us informed.


Thank you for your indulgence,

BassCliff
 
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