Smaller carbs for kz750-4

wrote:

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.
 

So I would need to make the holes oval on the carb boots because the bolt spacing is smaller on the kz650/750 head, is that correctly understood?

Yes.

No eBay APP ID and/or Cert ID defined in Kunena configurationThese would probably bolt right on. I believe they are 32mm.

 

No eBay APP ID and/or Cert ID defined in Kunena configuration

wrote:

These would probably bolt right on. I believe they are 32mm.

 

Thanks a lot. I think however I will find some 26-28mm if I decide to go that route. 2mm smaller will probably not be that big of a change. 

But thanks a lot for thinking about me and linking. That is really kind of you. 

wrote:

These would probably bolt right on. I believe they are 32mm.

Thanks a lot. I think however I will find some 26-28mm if I decide to go that route. 2mm smaller will probably not be that big of a change. 

But thanks a lot for thinking about me and linking. That is really kind of you. 

Do you have the factory pipes and air filter? Those are restrictive but good for low-mid torque.

Those Zephyr carbs would likely flow more  air than the Keihin's because they don't have choke flaps hanging in the airflow.

wrote:

wrote:

These would probably bolt right on. I believe they are 32mm.

Thanks a lot. I think however I will find some 26-28mm if I decide to go that route. 2mm smaller will probably not be that big of a change. 

But thanks a lot for thinking about me and linking. That is really kind of you. 

Do you have the factory pipes and air filter? Those are restrictive but good for low-mid torque.

No, not factory pipes. The header may be standard but I do not know. But the mufflers themselves are generic cafe racer style mufflers, which I baffled a bit to make them sound better. There is no balance/crossover pipe though, so I am considering trying that as a cheap remedy. 

The air filter is standard and I modified/cut away the long funnel/tube holding the air filter to allow more air coming into the air box. 

I feel like a lot of emphasis is being put on carburettor size and not much else. I mentioned earlier that CV carbs can act like smaller carbs at lower RPMs, so if you are interested in low and mid torque, be sure that they are replaced with smaller CV carbs - smaller slide carbs will not be an improvement.

But even if you find the 'perfect' carbs that have just the right airflow you're after at low and mid rpm, you still won't have good low/mid rpm torque. These engines have decent sized intake and exhaust ports, large pipes, and a fair amount of valve overlap, all designed to let more air and fuel in at higher rpm. The 34mm CV carbs that came with the engine are not a compromise, they are well suited to the engine, which is designed for mid-high rpm power while maintaining good ridability (though not good torque) at lower speeds.

I'm all for modifications - it's one of the best reasons to ride these bikes. But keep in mind that if you're after low-mid torque, you're better off starting out with a big twin designed for that criteria, rather than modifying a bike that wasn't. You'll be fighting every other design choice every step of the way.

Have you considered looking into a KZ750 twin?

The 34mm CV carbs that came with the engine are not a compromise, they are well suited to the engine, which is designed for mid-high rpm power while maintaining good rideability (though not good torque) at lower speeds.

Pretty spot on,although I think the torque is adequate for a street bike, i wouldn't say it's "not good" unless you're comparing it to a KZ1000, 1100.  Really, the only way you are going to get the sort of torque you seem to be chasing is to get a bike with a bigger engine, unless again, there is something inherently wrong with your setup... I'm currently riding my sons KZ650 and there is nothing wrong with the torque, using the gearbox correctly is the key, its great around town. 

wrote:

I feel like a lot of emphasis is being put on carburettor size and not much else. I mentioned earlier that CV carbs can act like smaller carbs at lower RPMs, so if you are interested in low and mid torque, be sure that they are replaced with smaller CV carbs - smaller slide carbs will not be an improvement.

But even if you find the 'perfect' carbs that have just the right airflow you're after at low and mid rpm, you still won't have good low/mid rpm torque. These engines have decent sized intake and exhaust ports, large pipes, and a fair amount of valve overlap, all designed to let more air and fuel in at higher rpm. The 34mm CV carbs that came with the engine are not a compromise, they are well suited to the engine, which is designed for mid-high rpm power while maintaining good ridability (though not good torque) at lower speeds.

I'm all for modifications - it's one of the best reasons to ride these bikes. But keep in mind that if you're after low-mid torque, you're better off starting out with a big twin designed for that criteria, rather than modifying a bike that wasn't. You'll be fighting every other design choice every step of the way.

Have you considered looking into a KZ750 twin?
 

I totally agree. And that is also my concern or dilemma. One of the reasons for having these great old bikes is to be able to tinker with them. Trying to modify the bike to fit my needs could be a fun learning experience. But it may also be a wasted effort, as it may never become quite as I want it, because that is not what it is designed for.

So yes, funny you mention it. I have very seriously considered the KZ750 twin. There was one up for sale here in Denmark some months ago that I really regret not buying. But yes, I am seriously considering a parallel twin. It seems to have more of the characteristics that I am after.

So my mind is not settled on anything yet, I am just researching the options.

The KZ750 twin has 55HP,and 44.8 ft-lbs of torque , the KZ 650 has 66HP and almost the same torque, the KZ750 four has 74HP  and about the same torque, {2 or 3 ft-lb difference}, you'd be going backwards with the twin...