Faced with a carb choice: Which would you choose.

[Big T]

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.

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

[tkent02]

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.

[FLATout?]

Why dont you sell both sets and get some Tm's or roundslides?

Now there'a a novel idea. For which size of either should I keep my eye out?

[FLATout?]

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.

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?

[trippivot]

BS 34 will run smoother and get better efficiency

[FLATout?]

BS 34 will run smoother and get better efficiency

Thanks! I'll use those, then.

[Big T]

While the BS carbs are larger, they give equivalent performance to the VMs

The differences between slide carbs and CV carbs causes the 26 VM/34 BS to give the same performance

[duaneage]

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?

Don't judge the carbs by the MM size. They operate differently so comparing them on bore size is not applicable.
I would go 34 MM CV and rebuild them carefully. If you have stock exhaust and airbox stick to the published specs but use a 42.5 pilot jet. The float levels need to be precisely set, I had drive-ability issues with mine until I set the floats correctly. I tried the 40 pilot but could not get it to idle properly. The air screws provide a range of adjustment but the pilot dictates the range to start with. With a 40 it just wouldn't settle down.

I'm running a VH street pipe with standard carb settings (except for the pilot jet) and contemplating a move from 115 to 117.5. The only effect will be on WOT above 8000 RPM which I probably shouldn't be spending much time at. This thing cruises at 85 mph around 6000 rpm as it is. I'm going to make a float level tester from clear tubing and an adapter to the fuel bowls to verify the actual levels. Measuring out on the bench is OK but nothing beats actually observing how high the fuel is in the bowls.

CV versus VM threads are like Oil and Tire threads. Ask 8 people and you get 10 opinions.