TOPIC:

I have the Keihin CV34 carbs, which works fine at high rpm, but I really want more low/mid range grunt. Hence I thought about finding some 24-28mm to replace them and see if I can gain some more grunt low down. 

Are there any from other models that are direct replacements? Or perhaps with replacing both intake rubbers and carbs will fit directly?

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.

wrote:

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.

Perhaps a larger rear sprocket ?

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Perhaps a larger rear sprocket ?
 

I have actually gone the other way to make it bearable on highways

So I am very specifically looking for recommendations on smaller carbs, that is either somewhat plug and play or which might be modified to work either with different intakes or otherwise.

The stock carbs are CV type, so the slides only open when there is demand from the engine side for more airflow.  I doubt you would pick up much if anything in terms of low-end power with smaller carbs.  

Do you have the stock airbox installed on your bike?  Running pod air filters will reduce low-end power.  

As mentioned in your other thread about  wanting more low-end power, an 810 kit is the way.  

VM24's from a KZ650 are a direct swap if you use the rubber manifolds for those as well. They have the same port to port spacing and the manifolds bolt right on.

In your profile pic the bike looks modified, if you aren't using the stock airbox you can space the carbs away from the head (longer intake tract boosts low range torque). Modern cars run variable manifolds, long at low revs for bottom end torque and short at high revs for max top end power. Smaller carbs will strangle the top end

wrote:

The stock carbs are CV type, so the slides only open when there is demand from the engine side for more airflow.  I doubt you would pick up much if anything in terms of low-end power with smaller carbs.  

Do you have the stock airbox installed on your bike?  Running pod air filters will reduce low-end power.  

As mentioned in your other thread about  wanting more low-end power, an 810 kit is the way.  
 

Yes, I run the stock airbox and I can only say that it runs as good as if it had dialed-in injection. But there just isn't enough vacuum at low rpm to get the carbs properly going. 810 kit is still on my radar, but even for that amount of cc I think 34mm carbs are too big for what I want to use the bike for.

Injected post=869897
VM24's from a KZ650 are a direct swap if you use the rubber manifolds for those as well. They have the same port to port spacing and the manifolds bolt right on. 

Ahh thanks a lot. I did not know that the KZ650 ran with that size of carbs. That sounds like a very interesting size and match. Then I should be able to get an airbox that worked as well.

Wookie58 post=869901
In your profile pic the bike looks modified, if you aren't using the stock airbox you can space the carbs away from the head (longer intake tract boosts low range torque). Modern cars run variable manifolds, long at low revs for bottom end torque and short at high revs for max top end power. Smaller carbs will strangle the top end 

Yes I run the stock airbox. But yes I agree with intake runner length. It could be worth a try to make them longer inside the airbox and see if it changes anything. Although adding length to the rubber attachments inside the airbox will probably be a challenge in terms of both space and viability. But thanks for planting the idea.

Wookie correct me if I’m wrong but longer intake would be the distance from the carb to head. Longer from the airbox to carbs won’t change anything. You can shorten the airbox boots and lengthen the carb holders. 

I am pretty sure that it is total length. It is also why pod filters can hurt the midrange as the total intake runner length becomes shorter and hence vacuum falls. 

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Wookie correct me if I’m wrong but longer intake would be the distance from the carb to head. Longer from the airbox to carbs won’t change anything. You can shorten the airbox boots and lengthen the carb holders. 

I believe the distance from head to throttle butterfly is more crucial than the length of the velocity stack although both do influence the torque curve. I am in no way an expert but I know in my car days "rally" cars used long manifolds and "circuit racers" used short ones. Don't forget exhaust dimensions also effect this (a friend's dad many years ago used to race British bikes and he used to select which exhaust he ran based on the length of the straights at that particular circuit) At the end of the day I would suggest a significant amount of research if you are trying to change the characteristics of an engines output and remember that everything is a compromise so gains at one end will equal losses at the other, this is why modern technology uses automatically variable manifold geometry and cam timing (the only way to have your cake and eat it)