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Faced with a carb choice: Which would you choose.

  • Thread starter Thread starter FLATout?
  • Start date Start date
F

FLATout?

Guest
Hello all! My project bike (seen here) came with a parts bike. This parts bike has ~23k miles, and so it's safe to assume the stock carburetors that are on it are VM26's.

Now, the main bike came with a full set of what I believe to be Mikuni BS34's (CV-types). Can someone confirm this? The inner diameter of the engine-side port is 34mm. See the pictures:

Carb pics

I've gotten these disassembled, cleaned, and have received the BS (CV) o-ring kit from CycleOrings. I'm following the guide to tear down and rebuild found here.

My question to you is: BS34 or VM26?
 
Depends on how "original" you want to keep the bike. Both will fit and work well. Just swap the intake manifolds around that fit the CVs if thats the way you decide to go.
 
Some people prefer the CV carbs as they say they're easier to tune and will adjust better to elevation changes than VM carbs. This may be true but it's pretty fun tuning VM carbs sometimes (some may say that's just plain crazy talk) and they are just more 'retro' :p
 
Opinions will vary. For me, the VM carbs were the reason I sold my 78 750E. If you nail the throttle it falls flat on its face before taking off. The CV carbs solve that issue. But the VMs are easier and cheaper to tune, you just have to learn to roll into the throttle rather than nailing it.
I prefer the CVs for my riding style.
 
I like VMs when I ride in a narrow range of elevation. They are easier to tune. I like CVs for mountains. Nice to be able to go up to 14,000ft and down to sea level without having it run crappy.

Around here it's a no brainer. CVs all the way.
 
Kewl. I'm in the Seattle area, so rides through the Cascade mountain range will be on the reg. CV seems to be the way to go for mountain rides.

Followup question: I need to rejet these for pod and a 4-1, and was thinking of a Dynojet kit. However, I don't think these carbs were stock on the GS1000. How do I make sure I grab the correct Dynojet kit?
 
FLATout? said:
Kewl. I'm in the Seattle area, so rides through the Cascade mountain range will be on the reg. CV seems to be the way to go for mountain rides.

Followup question: I need to rejet these for pod and a 4-1, and was thinking of a Dynojet kit. However, I don't think these carbs were stock on the GS1000. How do I make sure I grab the correct Dynojet kit?
Click to expand...
Buy the correct Stage 3 kit for the carbs. You can adjust them for the proper AFR by adjusting the size of the mains ($8 a set from DynoJet), raising/lowering the needles, then tweaking the air/fuel mixture screws (the ones under the little caps).
Study up on carb theory and carb tuning whilst waiting on the kit.
Some good places to start:
CV Carb Operation
General carb theory and tuning
CV Carb Tuning Procedure
 
All my bikes ( Suzukis ) have VMs and they dont fall off. The only bike that has CVs is the 73 Yammy and its no different in my opinion than the way the VMs react.

Maybe it was the "little on the leaner side" they had set them for EPA bullcrap.
 
FLATout? said:
Kewl. I'm in the Seattle area, so rides through the Cascade mountain range will be on the reg. CV seems to be the way to go for mountain rides.

Followup question: I need to rejet these for pod and a 4-1, and was thinking of a Dynojet kit. However, I don't think these carbs were stock on the GS1000. How do I make sure I grab the correct Dynojet kit?
Click to expand...

I never have an altitude issue in the Cascades, the passes and roads just aren't that high
 
chuck hahn said:
All my bikes ( Suzukis ) have VMs and they dont fall off. The only bike that has CVs is the 73 Yammy and its no different in my opinion than the way the VMs react.

Maybe it was the "little on the leaner side" they had set them for EPA bullcrap.
Click to expand...

When was the last time you rode at 8-10,000'?

I'm guessing never
 
Big T said:
I never have an altitude issue in the Cascades, the passes and roads just aren't that high
Click to expand...

Well they won't be unrideable, but at 4,000 - 5,000 ft the VMs, if tuned for sea level, will not be working as well as the CVs. You will have to keep the engine wound up higher, need to feather the throttle at times. Above that they get worse and worse.

You can remove the airbox lid to give a temporary lean mixture for higher elevation. It will not be perfect but it helps.
 
Correct me if i'm wrong, but it appears your bike is a 1980 GS1000ET (slotted rotors, rectangle master cylinder) and actually a rare-ish model. If this is the case, it came stock with BS34CV's (at least US models did). Therefore, 26mm carbs won't readily fit into your intake manifolds without adapters... and good luck finding those; probably be easier to mill a set... or use '78/'79 manifolds and slot the flange mounting holes.

If it is in fact an '80 model, you'll want to stick with the larger CV carbs.

Nice motorcycle btw.
 
tkent02 said:
Well they won't be unrideable, but at 4,000 - 5,000 ft the VMs, if tuned for sea level, will not be working as well as the CVs. You will have to keep the engine wound up higher, need to feather the throttle at times. Above that they get worse and worse.

You can remove the airbox lid to give a temporary lean mixture for higher elevation. It will not be perfect but it helps.
Click to expand...

Tom,

I generally don't find much problem up to 6,000'. That being said, I don't spend much time up there

When I was at a rally in Reno, I definitely noticed the bike was weak in the knees
 
Weak, and full throttle gives too much gas, you have to keep it reved up higher and use less throttle. Way up high you go full throttle and it falls on it's ass, and above that it won't even run.

The CVs just keep going fine, they get a little less powerful as elevation goes up but you can ride it normally, it's no big deal. Sea level jetting works fine, except for the pilot circuit.
 
Frank Z. said:
Correct me if i'm wrong, but it appears your bike is a 1980 GS1000ET (slotted rotors, rectangle master cylinder) and actually a rare-ish model. If this is the case, it came stock with BS34CV's (at least US models did). Therefore, 26mm carbs won't readily fit into your intake manifolds without adapters... and good luck finding those; probably be easier to mill a set... or use '78/'79 manifolds and slot the flange mounting holes.

If it is in fact an '80 model, you'll want to stick with the larger CV carbs.
Click to expand...

Thanks to you Frank Z, I've been having a hard time identifying the exact model, however there's a chance that none of the parts on my bike came stock. It was kinda a conglomerate of three separate parts bikes, but to what extent I'm not aware.

In terms of the size difference between the VM26 and the BS34CV carbs. I've read that going to bigger carbs increases performance. In the article I'm using as a mod guide, the author says that going with 29mm 'smoothbore' carbs made all the difference. To me it seems that 34mm carbs are almost...too large. Being of a stock selection, will these large carburetors deliver a noticeable power gain over, say the VM26's?
 
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