What are the differences between VM, BS & CV carbs?

[GS750]

Hi all, just wondering if someone could give me a quick run down on these 3 styles of carbs, and their pro's and cons.

VM, BS & CV's



I have a GS750B fitted with an 850bb wiseco kit, and just got to thinking, maybe I should replace the VM26 carbs with the CV's from the 850G??

I read in the first edition Haynes GS750 manual today (in the performance section) that they suggested a change to VM29's on the modified GS750's, especially those with the 850 or larger bigbore kit, AND that it would increase not only performance, but mileage as well.

Any thoughts on this subject would be appreciated please.

 

[Big Rich]

CV stands for constant velocity- and BS carbs are CV carbs. When the throttle is twisted it lifts th butterfly valves, causing the vacuum from the pistons to lift the slide under the diaphragms, which draws more air and fuel into the chamber.

VM carbs are Mikuni's "non-CV" carbs. The throttle is linked to the slides directly from a cable. You twist, they lift, engine revs. Simple.

 

[GS750]

so which ones are the better option?? I cant seem to find ANY VM29's ANYWHERE :'(

 

[Big T]

VM 29 (smoothbore) carbs are rare and expensive

Chuck just had a set up for sale at $450, Bruce sold a set for over $1,000

The BS carbs, while larger, do not give any performance boost compared to the VMs due to the differences in design

Plus, you should switch to the 80 or later 850 head to accomadate the BS carbs

33 mm slide carbs would be easier to find, but may be too big for your set up

 

[TheCafeKid]

You can't use BS (CV) carbs with your current head unless you could manufacture some intake boots that would work.

Cv type carbs are generally larger mouthed than mechanical carbs and therefore won't simply swap. In '80, on this side of the pond, ALL factory motorcycles nearly had switched over to CV carbs to meet increasing emissions standards. But just because they're bigger bores doesn't mean they're better.
I'm not sure when/if you guys were forced to switch. They may have continued to use VMs for a while there. I dunno.
If you REALLy wanted to, you could do as BigT said and swap to an 80 850 head but it requires some work as the cases will need a little grindig for it to sit properly. It's been done before on here. I'm sure a search on the subject would bring up a thread.
FWIW I believe the 77-79 750 head and the 79 850 head were the same part #

 

[Steve]

I have a GS750B fitted with an 850bb wiseco kit, and just got to thinking, maybe I should replace the VM26 carbs with the CV's from the 850G??

You have already gotten the answer to the different types, so I will pass on that.

Just so you know, the '79 850 also came with VM26 carbs, so the only thing you might have to address would be the jetting to match your engine.

.

 

[GS750]

You have already gotten the answer to the different types, so I will pass on that.

Just so you know, the '79 850 also came with VM26 carbs, so the only thing you might have to address would be the jetting to match your engine.

.

Thanks for the reply's everyone!

I noticed that some 1000's had the VM26's but surely the VM29's would flow more air?

I also know first hand through 20+ years of mechanical work, that loads of things have been done to internal combustion engines of all sorts, from small garden product engines to large automotive engines, so that they comply with regulations such as emissions and noise.

Things such as inlet restrictors and strangled exhausts have been a common alteration so that a vehicle complies with these standards which vary from country to country, and even state to state.

My recent thoughts were (as I dont have a large workshop, flow bench, and testing facility) since I have 850'd my 750 engine, improved its exhaust, ignition, and even the fuel I run (only run 98 octane), then surely improving the inlet would be a sensible thing to do, and would remove a restriction on flow that the VM26's would surely have, without going over the top on its flow ability's.

 

[TheCafeKid]

98?? What compression ratio are you running after the bore kit? If it's not like 10:1 or greater you're wasting your money. And not doing your engine any good. 87 is perfectly fine.

 

[GS750]

with the head work etc, it is 10.2:1 comp,

 

[Steve]

I noticed that some 1000's had the VM26's but surely the VM29's would flow more air?

Yes, they can flow more air.

The larger question is, will your 850 move that much air? Likely not, unless you have done a bunch of other mods.

Using carbs that are too big will affect the velocity of the air through them, which will greatly affect how well they carburate.
It is far better to keep the velocity up where it works better.

My recent thoughts were (as I dont have a large workshop, flow bench, and testing facility) since I have 850'd my 750 engine, improved its exhaust, ignition, and even the fuel I run (only run 98 octane),

98?? What compression ratio are you running after the bore kit? If it's not like 10:1 or greater you're wasting your money. And not doing your engine any good. 87 is perfectly fine.

Josh, you have to remember that they measure octane a bit differently than we do. Not sure exactly how they compare, but their numbers will be several points higher than ours.

There are two methods used to measure gasoline's resistance to "knocking", the Research method and the Motor method. Our octane is assigned a number that is (R+M)/2. Theirs is simple R or M, I don't remember which.

.

 

[GS750]

We rate fuel as RON (research octane number) over here.

that is 98 ron that I run

Some country's I notice use MON (Motor octane number)

 

[GS750]

Our octane is assigned a number that is (R+M)/2. Theirs is simple R or M, I don't remember which.

.

So our 98 is your 93 then lol