Upgrade carbs for KZ750

Hi all,

I'm looking for alternative (and possibly an upgrade) for the carbs on my KZ750.
I currently have Keihin CV34's which have been a pain for ever.
They have been cleaned a million times with varying success (did it so often that I could eventually pull the carbs in <5 minutes).
The bike ran reasonable on a lot of occasions, but was never completely happy. Leave it standing for a month and the troubles would come back in one form or another.
Last time i saw that the slides are deteriorating now as well. 
With that, i'm done and want to fix it for good now.

But where to go?

As for the bike:

• The exact type is vague. Registration states KZ700A1, but there is a 750cc cilinder on it which has been rehoned recently.
• The head features exhaust air injection on the valve covers (should that help) and is rebuild together with the cilinder hone (new valves, seats milled, new seals, guides were good).
• It has a new (reproduction) kerker megaphone 4 into 1.
• Coils are replaced with Dynatec 2.2Ohm coils (grey) on the stock igniter box
• Still has the airbox. Not sure if i want to keep it. Will depend on carb choice as well.
• Oversized bore is not on the table for now, maybe when the current setup wears out.

Now the main topic, what to do with my carbs?

I would like a nice 'race' inspired look eventually, and a nice set of carbs would complete that.
I am planning on doing some trackdays later this year, but general (spirited) driving is the main item.

• Obvious swap would be another set of CV34's or BS34's. But i fear i'm looking at the same level of wear and rot as i do now.
• Mikuni thinks RS34's are awesome for this, and they look fabulous. But the general opinion seems to be that this is way too big for this engine.
• I came across people mentioning Mikuni BST36 (suzuki slingslot) carbs, but little details
• CV34's from a ZR7S are also mentioned as slight modernization
• CR specials (29mm i think?) could be, although i like the blunt flatslide look more. And they seem to be a lot more expensive.
• VM26SS could be a budget friendlier alternative to the CR's
• Mikuni TMR28 is offered by Topham, at a price

Lots of options, little concensus on the right path to take.

What would be options to avoid? What are good combinations?


There seems to be little availability here (NL), so i'll likely have to source parts abroad. 

Hi Timberleek,
  An old friend of mine had a new 1981 kz750 with the vacuum stock carbs. He bought a year old head to have set up for a mr. turbo . Before he sent out the head someone he knew told him to try mikuni smoothbores 29's.

   He did, and NEVER LOOKED BACK !!! 
The head set in the basement for years and he ended up giving it to me.
   He started out with stock jets, pilots and by the smoothbores design it litt up !
   They are not cheap but they Work & Work well simply. 
    Just a thought.

I like the CV34's, having rebuilt a number of sets.  Of course, the Mikuni BS34's are also nice.  The only real weakness they have is in terms of pods support, maybe.  Even that can be dealt with, though.

I don't have any experience with RS34's, but they seem worth a try.  Maybe someone else here will have direct experiences with them on a 750.

Overall, in terms of original type carbs, 90% of the problems come from people that don't do a proper and complete rebuild.  I'm not saying this is the case with you, but I've seen it too many times to count.  People try to "rebuild" the set without unganging the carbs.  A slow and detailed teardown is actually the fastest way to success.  Anyway, good luck...

P1020763 by nessism , on Flickr

P1030216 by nessism , on Flickr  

I converted my kz750 to cvk32’s that came from a zx600a/b/c which is the first generation Ninja 600 (otherwise known as the gpz600r everywhere else in the world) and were also used on the Zephyr zr750 bikes. They are exactly the same as the zr-7 cvk32’s except the zr-7’s came with an integrated throttle position sensor.  The spacing and inlet and outlet dimensions are all the same for all the cvk carbs of that vintage. They just used different intake boots to account for the difference in cylinder spacing. They only came with 30mm or 32mm bores and different pressed in air jets for the numerous applications they were used for. I chose the zx600 ones simply because they have the exact air jets as the Zephyr and the zr-7 and were a fraction of the price. 

To convert, all you will need is the rubber intake boots for a zephyr 750 or the zr-7 , the airbox inlet boots (unless you do pods) a push/pull throttle (or not if you want to live dangerously) and a 750 jet kit from Factory Pro (doesn’t matter what bike they come off of, they all need to be re-jetted as factory tunes were not great to say the least) 

My personal setup is zx600 carbs, pod filters, Factory Pro Stage 2 jet kit and Zephyr throttle assembly.  I was forced to run pod filters due to the custom nature of my bike and it took a bit to get them tuned correctly. But once I solved the tuning issue, the carbs simply function as flawlessly as if it was fuel injection. No bogs, flat spots, backfires or surging. 
I have always wanted a set of fcr flat sides or cr29 specials, just because I love their functionality and design. But I don’t race my bike or spend any of my time in the upper rpm’s, and that’s where those carbs really shine. That and they also cost 4x what I spent on my cvk swap. 
The rs34’s are also very clever carbs, but are simply way too big for even the most hopped-up 750. 
If you are interested, more carb info can be found in the build thread for my bike. The link is in my bio. 

 

 

More modern CVK32 carbs seem to be the best choice (ZR7S and early Ninja 600) and you can still use the stock air box with a slight mod. Lots of KZ650/750 owners have switched over to them without too much drama.

The CR29's are a excellent carb if you can afford them plus they seem to be just the correct size for a stock 750.

The RS34's will work but flow way more air than a stock 750 engine can utilize efficiently.

I can attest that the CVK carbs run almost as good as fuel injection once set up properly.
 

I have also switched to CVK32 carbs off an early Ninja 600.
Followed the tutorial by Pablo at the 650proboards site.
Smooth running, half the weight of stock carbs, and 20 year advancement in carb design.
 

rstnick wrote:

I have also switched to CVK32 carbs off an early Ninja 600.
Followed the tutorial by Pablo at the 650proboards site.
Smooth running, half the weight of stock carbs, and 20 year advancement in carb design.

 

Hey Rob, more and more people are looking into this mod - maybe you could post a link ?

Wookie58 wrote:

rstnick wrote:

I have also switched to CVK32 carbs off an early Ninja 600.
Followed the tutorial by Pablo at the 650proboards site.
Smooth running, half the weight of stock carbs, and 20 year advancement in carb design.



 

Hey Rob, more and more people are looking into this mod - maybe you could post a link ?

 

You need to be a member before you can access anything other than the home page. Its only takes a couple of days to become a member once registered.

Here is the link :
z650.proboards.com/thread/3117/installin...buretors-z650?page=1

There is no PDF but a comprehensive tutorial with pictures, there are 7 pages so far with lots of different examples of owners who have made the switch to CVK's.

 

Sorry, it took a bit longer to respond.

Thanks for all of the tips.

Good to know that the GPZ600 has the same carbs as the ZR7, those are a lot easier to obtain around here.
I would like some fancy carbs, but that comes at a price. Maybe i'll start with the CVKs first and get her finally (hopefully) running right.
Then postpone the rest for if/when i do more performance mods.

I've subscribed to z650.proboards. Now just some waiting for the approval to get through.

I'll post an update when i have some results

I was also looking for some carbs for my 750 engine swap into my KZ650 frame since the motor did not come with any carbs. I considered OEM style CV34's and some even the RS34's I already had. I ended up using some CVK34's I bought cheap from a 1990 Ninja 750 (not sure if this is the same swap as the CVK32's)

The CVK's are more performance orientated than earlier CV's because they incorporate a hybrid "flat slide" design which gives better fuel pickup and increases CFM potential. All CV's have way better low speed mixture as compared to old round slide carb designs. 

From noting these improved design considerations I would not doubt a set of CVK carbs could handle any high performance upgrade you could throw at them (cams/porting/big bore etc)
I have even seen CVK's on a 1428cc KZ based Pro Street bike at a local bike show :

 

 

Daftrusty, side note, your bike is really clean!

@Daftrusty & rstnick,

I've obtained a set of GPZ600R carbs and started looking into parts differences to the ZR7 (ideal order of operations, i know).
The boots are obvious, i need those anyway.
But What internals did you use?

Looking at the replacement part numbers for both bikes. Almost everything seems to be different.
Slides, needles, emulsion tubes and even floats and float needle
With the time difference between the bikes, i can imagine some numbers are different due to negligible production differences.

Did you just change to the proper needles (N6NC & N4MD) and jets (35/108 stock zr7, 38/108 stock ZX600 and 45/112 seems common for the kz650 jobs)?