Smaller carbs for kz750-4

The CV34's have butterfly valves blocking airflow, so even though the outlet is 34mm, they don't flow like a slide carb would at 34mm.  How much the butterfly cuts down on airflow, I'm not sure, though.

By the time you buy some VM24's, rebuild them, buy new intake boots, maybe a new airbox too, or at least new boots, you could have made the jump to 810.  An 810 kit with a Daftrusty 6 speed transmission conversion will be the best of both worlds, since you can run a low first gear for launch, and a high 6 speed for low rpm cruising.  Go BIG or go home! 

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Basically carbs are designed to give you a roughly 15/1 air/fuel mixture across the range.  A little bit richer at low rpm and a little bit leaner at cruising rpm.  The bottom line here is that smaller carbs are not the way to go if you want more power at low rpm.  You will be starving the engine of air, and because you want to maintain that air/fuel ratio you'll be starving your engine of fuel.  If you're having problems with your engine power at low rpm you can play a bit with the pilot circuit on your existing carbs to see if that helps.  Perhaps they just need to be cleaned and re-synched.

 

Thanks a lot for your answer, but I will politely disagree. Carbs works by vacuum and are designed to work within a "window" of vacuum. One of the reasons why there is a lack of grunt in the low-midrange rpm is that 34mm carbs are really big for 750cc and they are that size to deliver maximum power at high rpm. The reason for them not delivering enough grunt down low, is that there is not enough vacuum for them to properly work until late in the rpm range.

And it is not a question of them not being adjusted, synced or otherwise as they are properly tuned and jetted, they are just not suited for low-midrange performance on a 750cc bike.

So I appreciate the feedback, but I am looking for smaller carbs in order to have then work at lower vacuum and hence at lower rpm. It might sacrifice some top end performance, but that is a trade-off I am willing to take.

I have a set of VM 24ss and VM28ss pumpers (1980) that I would be prepared to sell.  I also have a spare set of VM28 SC's (1974/75 Z1) but they're not for sale.  The 24's will fit without modification.  The 28's are wider spaced and would require modifications.

One of the reasons why there is a lack of grunt in the low-midrange rpm is that 34mm carbs are really big for 750cc 

And it is not a question of them not being adjusted, synced or otherwise as they are properly tuned and jetted, they are just not suited for low-midrange performance on a 750cc bike.

CV, or Constant Velocity carbs offer the best real-world performance for day to day riding where your throttle isn't pinned all the time. the CV carb allows much more precise metering and thus, a less boggy engine under load conditions.
Keihin 34 CV's are NOT really big on a 750, you can't look at the number and come to that conclusion. A 34mm CV is equivalent to a 26 to 28 mm slide carb, the CV works on vacuum and the slide carb works by manually raising the slide by cable, two entirely different things. The keihins wouldn't be referred to as a "performance" carb either, they're no where near as performance orientated as the Mikuni BS34's for example {which I have on my 750}. If you are suffering a lack of grunt, go ever you're whole induction system.
What airbox/filter are you running ? If you made changes, what did you do to the carbs to compliment the changes ?.... Got good spark ?  Resister plugs or leads ?  timing ? What have you checked ?
Your second part of that statement is just plain wrong,do you know just how many 650-750 bikes use that sized CV carb ? ...

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One of the reasons why there is a lack of grunt in the low-midrange rpm is that 34mm carbs are really big for 750cc 

And it is not a question of them not being adjusted, synced or otherwise as they are properly tuned and jetted, they are just not suited for low-midrange performance on a 750cc bike.

CV, or Constant Velocity carbs offer the best real-world performance for day to day riding where your throttle isn't pinned all the time. the CV carb allows much more precise metering and thus, a less boggy engine under load conditions.
Keihin 34 CV's are NOT really big on a 750, you can't look at the number and come to that conclusion. A 34mm CV is equivalent to a 26 to 28 mm slide carb, the CV works on vacuum and the slide carb works by manually raising the slide by cable, two entirely different things. The keihins wouldn't be referred to as a "performance" carb either, they're no where near as performance orientated as the Mikuni BS34's for example {which I have on my 750}. If you are suffering a lack of grunt, go ever you're whole induction system.
What airbox/filter are you running ? If you made changes, what did you do to the carbs to compliment the changes ?.... Got good spark ?  Resister plugs or leads ?  timing ? What have you checked ?
Your second part of that statement is just plain wrong,do you know just how many 650-750 bikes use that sized CV carb ? ...

I think this is a man with a mission.  He wants to make his own mistakes.
 

I won't pretend to be an expert on these carbs and these bikes, but I do agree with what Nessism and 750 R1 have said. CV carbs are specifically designed to act like a smaller carb at lower rpm, and a larger carb at high rpm. The slide is not connected to the throttle cable, it is vacuum operated, and when the vacuum is low the slide will remain down, effectively reducing the size of the carb intake. They're called Constant Velocity carbs because they keep air speed high at low and high rpms.

Slam the throttle open on a 34mm slide carb at low rpm, the cable will pull the slide up, opening the carb up to its full 34mm, and the air speed will be slow and torque will be poor until the engine gets to a higher RPM.

Slam the throttle open on a 34mm CV carb at low rpm, the butterfly will open, but the slide will remain down while the rpms are low due to the low vacuum. This keeps the intake narrow - like a smaller carburettor - until the rpms pick up, increase the vacuum, and the slide will open further as the vacuum increases, until it opens all the way when the engine is drawing as much as possible. Air speed will remain high throughout the range.

Switching to a smaller slide carb may give marginally better performance at a lower rpm (similar to how switching to a large slide carb can give marginal performance at high rpm while sacrificing mid and low-end torque of a CV), but the problem is that it will be perfect at a certain vacuum, and performance will be poor as you move further above or below that level. That means that you might get good mid-range torque, but both your high and low range torque will suffer. CV carbs are specifically designed to avoid that issue that slide carbs have, they are 95% as good as a smaller slide carb at low rpms, and 95% as good as a large slide carb at high rpms. People are often changing out CV carbs to slide carbs on race/drag bikes because that extra 5% at high-rpm full throttle is more important than the gains made down low by the CV carbs.

If mid-range torque and ridability is what you're after, switching to smaller CV carbs might see some minor gains in low rpm torque at the expense of high rpm torque. But switching to smaller slide carbs like the VMs you've mentioned will likely improve performance at one point through the rev range, and performance will suffer everywhere else.

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The CV34's have butterfly valves blocking airflow, so even though the outlet is 34mm, they don't flow like a slide carb would at 34mm.  How much the butterfly cuts down on airflow, I'm not sure, though.

By the time you buy some VM24's, rebuild them, buy new intake boots, maybe a new airbox too, or at least new boots, you could have made the jump to 810.  An 810 kit with a Daftrusty 6 speed transmission conversion will be the best of both worlds, since you can run a low first gear for launch, and a high 6 speed for low rpm cruising.  Go BIG or go home! 

 

The cost argument is of course a very valid one! You are probably right that I can do the 810 conversion for very close to the same cost of smaller carbs - so good point.

I have to read up on that 6 speed conversion, that sounds very interesting!

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One of the reasons why there is a lack of grunt in the low-midrange rpm is that 34mm carbs are really big for 750cc 

And it is not a question of them not being adjusted, synced or otherwise as they are properly tuned and jetted, they are just not suited for low-midrange performance on a 750cc bike.

CV, or Constant Velocity carbs offer the best real-world performance for day to day riding where your throttle isn't pinned all the time. the CV carb allows much more precise metering and thus, a less boggy engine under load conditions.
Keihin 34 CV's are NOT really big on a 750, you can't look at the number and come to that conclusion. A 34mm CV is equivalent to a 26 to 28 mm slide carb, the CV works on vacuum and the slide carb works by manually raising the slide by cable, two entirely different things. The keihins wouldn't be referred to as a "performance" carb either, they're no where near as performance orientated as the Mikuni BS34's for example {which I have on my 750}. If you are suffering a lack of grunt, go ever you're whole induction system.
What airbox/filter are you running ? If you made changes, what did you do to the carbs to compliment the changes ?.... Got good spark ?  Resister plugs or leads ?  timing ? What have you checked ?
Your second part of that statement is just plain wrong,do you know just how many 650-750 bikes use that sized CV carb ? ...
 

There is no need to go into an argument on whether 34 mm carbs is fitted to many 650-750 bikes. I am sure they are, but they are there to offer top end performance, which is totally fair. But that is not what I am after.

There is a nice comparison of 32, 34 and 36mm carbs fitted to a z900 in the "4 stroke performance tuning" book. And although the 36mm offered most power up top, they lacked a lot in the midrange where I spent most of my time. So I am not trying to claim some universal truth about performance I am just explaining what I am after.

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I won't pretend to be an expert on these carbs and these bikes, but I do agree with what Nessism and 750 R1 have said. CV carbs are specifically designed to act like a smaller carb at lower rpm, and a larger carb at high rpm. The slide is not connected to the throttle cable, it is vacuum operated, and when the vacuum is low the slide will remain down, effectively reducing the size of the carb intake. They're called Constant Velocity carbs because they keep air speed high at low and high rpms.

Slam the throttle open on a 34mm slide carb at low rpm, the cable will pull the slide up, opening the carb up to its full 34mm, and the air speed will be slow and torque will be poor until the engine gets to a higher RPM.

Slam the throttle open on a 34mm CV carb at low rpm, the butterfly will open, but the slide will remain down while the rpms are low due to the low vacuum. This keeps the intake narrow - like a smaller carburettor - until the rpms pick up, increase the vacuum, and the slide will open further as the vacuum increases, until it opens all the way when the engine is drawing as much as possible. Air speed will remain high throughout the range.

Switching to a smaller slide carb may give marginally better performance at a lower rpm (similar to how switching to a large slide carb can give marginal performance at high rpm while sacrificing mid and low-end torque of a CV), but the problem is that it will be perfect at a certain vacuum, and performance will be poor as you move further above or below that level. That means that you might get good mid-range torque, but both your high and low range torque will suffer. CV carbs are specifically designed to avoid that issue that slide carbs have, they are 95% as good as a smaller slide carb at low rpms, and 95% as good as a large slide carb at high rpms. People are often changing out CV carbs to slide carbs on race/drag bikes because that extra 5% at high-rpm full throttle is more important than the gains made down low by the CV carbs.

If mid-range torque and ridability is what you're after, switching to smaller CV carbs might see some minor gains in low rpm torque at the expense of high rpm torque. But switching to smaller slide carbs like the VMs you've mentioned will likely improve performance at one point through the rev range, and performance will suffer everywhere else.
 

Thanks a lot for the long response. I agree that CV carbs are trying to mimic smaller carbs. It is of course not perfect, but I agree on your overall arguments.

My original question was about which smaller carbs that could fit and there has been a good discussion about whether this is the right way to go or not - which is a fair and good discussion.

I think the cost argument around adding CC's is a good one. I have also looked a bit into adding a crossover pipe to the exhaust and see if I gain something in the low-/midrange. That will only cost me a little pipe and a bit of welding.

but they are there to offer top end performance, which is totally fair. But that is not what I am after.

Sorry but that assumption is wrong,  CV's were designed to offer better overall performance right throughout the rev range. I would again argue that all the "performance" or "race" carbs of the era were slide carbs, and they were, I've owned a few of them like CR31's and VM 29 smoothbores, the RS 34 Mikuni's are another popular performance carb.  Slide carbs were the performance  option right up until fuel injection took over.  The shop where I work on my bikes is full of vintage 4 cylinder race bikes, none have CV's..
You didn't answer the question about your intake system or any of the other factors involved in proper tuning... I'd be curious to know how your bike is set up...
Not trying to argue here, just trying to clear up a few misconceptions...

My point was not that CV’s are there to offer top end. My point was that 34mm carbs on a 650-750 4 cylinder so each 34mm carb fueled 160-190cc was for the top end. We agree on why CV’s are used. 

I am running the stock air box with most of the long funnel that screws down into the filter cut away. Standard air filter. 

It has electronic ignition and runs 4-2 exhaust. As mentioned in one of the previous posts it does not have a crossover pipe between the two, which might be a cheap easy thing to test. 

I have a 2001 zr7S which uses the '82 750 engine. It uses the Keihin CVK 32 carbs as opposed to the 34 used in 82. 
The bike pulls fine from low rpm but other owners have increased the jetting to give it a little more snap off idle. I left mine stock. It fuels perfectly.

Another option is the 26mm vm carbs off a KZ900/1000. Use the carb boots for the carbs and elongate the bolt holes. Because the bolt pattern is smaller on the KZ650/750 head.
I have run vm24s on a 750 before and throttle response is peppier over the 34cv carbs. Probably because you're up on the needle earlier. But you lose 5 to 10 h.p. up top. With vm26s you'll break even.