Smaller carbs for kz750-4

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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...

The outright torque number is uninteresting. The interesting point is the torque curve and where in the rev range torque is available. The kz750-4 sure has good torque, but it is way up in the rev range. 

Unfortunately I haven’t seen many torque graphs for the 750 twin or for that matter the other parallel twins of the era like XS650 and T140.

How difficult to stuff a 1000 in that frame?

wrote:

wrote:

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...

The outright torque number is uninteresting. The interesting point is the torque curve and where in the rev range torque is available. The kz750-4 sure has good torque, but it is way up in the rev range. 

Unfortunately I haven’t seen many torque graphs for the 750 twin or for that matter the other parallel twins of the era like XS650 and T140.




If you are interested in the KZ750B search for the post of Doctor Rot and in the archive for Bounty Hunter and Steell.


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Did some one say modified KZ750 twin??!!!

Stock KZ750B about 47-50 WHP
light port and polish, RS34 carbs, digital ignition, full exhaust, bored 1mm over with close to stock compression - 55 WHP
light port and polish, RS34 carbs, digital ignition, full exhaust, high comp pistons (stock bore), custom cams - 62 WHP
currently working on a big bore engine with oversized titantium valves - 906cc hoping for 80-85 WHP

I don't remember the torque because that was never my focus, these bikes have plenty of torque, but lack mid to top end power IMO. The factory dyno numbers are not accurate but they are a good apples-to-apples comparision.

wrote:

wrote:

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...

The outright torque number is uninteresting. The interesting point is the torque curve and where in the rev range torque is available. The kz750-4 sure has good torque, but it is way up in the rev range. 

Unfortunately I haven’t seen many torque graphs for the 750 twin or for that matter the other parallel twins of the era like XS650 and T140.

If you look at the graphs Doc posted, the KZ650's torque starts just above where the 650's  is at,  a couple of hundred revs higher, where  the twin virtually stays the same, the 650's keeps climbing , I would put my money on the 650 being faster everywhere than the 750 Twin, with 100cc less, the 750/4 is faster again.  I'm riding my sons KZ650 every day and it has ample torque, not as much as my old GSXR1100, or any of the other {around 30},  larger bikes I've owned, but plenty for the street , and I don't have to rev the balls out of it to use it either, it would eat those heavy old twins for dinner, the KZ750/4 has more power again. How many of these bikes have you  owned  or ridden ?  

Look at Docs modified figures for the twin below, after all that work, my 1982 GPZ 750 has virtually the same HP stock, with a few tweeks , and a lot less money, i'll be around 100HP..
You could put a set of Cruizin image 810 pistons, around $180 dollars,a second hand set of later GPZ cams and you're instantly blowing the doors of the old twin for very little money. I know what I'd do.....

www.cruzinimage.net/2017/08/25/80-85-kaw...re-pistons-kit-69mm/

How many KZ750 twins have you owned?

I've had several KZ650s and a KZ750E and a modified KZ750 twin is more fun than either IMHO. In fact -  I even prefer it to my KZ1000 for in-city driving. My friend just sold his pristine KZ650 for my modified KZ750twin because he likes it that much more.

Everyone wants different things in bikes. its foolish to assume we all want balls-out performance. Otherwise why even bother with KZ's when you can go buy a clapped out R1 for $3k and blow the wheels off any streetable KZ built to nearly any level? 

I think you're missing the point here

wrote:

wrote:

wrote:

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...

The outright torque number is uninteresting. The interesting point is the torque curve and where in the rev range torque is available. The kz750-4 sure has good torque, but it is way up in the rev range. 

Unfortunately I haven’t seen many torque graphs for the 750 twin or for that matter the other parallel twins of the era like XS650 and T140.

If you look at the graphs Doc posted, the KZ650's torque starts just above where the 650's  is at,  a couple of hundred revs higher, where  the twin virtually stays the same, the 650's keeps climbing , I would put my money on the 650 being faster everywhere than the 750 Twin, with 100cc less, the 750/4 is faster again.  I'm riding my sons KZ650 every day and it has ample torque, not as much as my old GSXR1100, or any of the other {around 30},  larger bikes I've owned, but plenty for the street , and I don't have to rev the balls out of it to use it either, it would eat those heavy old twins for dinner, the KZ750/4 has more power again. How many of these bikes have you  owned  or ridden ?  

Look at Docs modified figures for the twin below, after all that work, my 1982 GPZ 750 has virtually the same HP stock, with a few tweeks , and a lot less money, i'll be around 100HP..
You could put a set of Cruizin image 810 pistons, around $180 dollars,a second hand set of later GPZ cams and you're instantly blowing the doors of the old twin for very little money. I know what I'd do.....

www.cruzinimage.net/2017/08/25/80-85-kaw...re-pistons-kit-69mm/


You forgot about the cost of boring out the cylinders.
Around here it's $80 a hole.
And since the head is off a valve job and reshim.
......... Damn, where did that rabbit hole come from?
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I don't think there's any real question about whether the i4 or twin is faster - the i4 is simply faster (and if it were all about speed, none of us would be riding old KZs). And yes, they both make the same peak torque.

But if you're after low-mid torque - specifically what this thread is about - the twins have it in spades. The i4s are only just waking up at 3000rpm, and start to really pull over 5000rpm. The twins will pull out of corners nicely at 2000rpm.

I personally prefer the feeling of knocking it back a gear and shooting off at 6000rpm that you get with the i4, but I understand why some prefer the feeling of crusing just above idle and pulling away with no effort in the twin.

wrote:

wrote:

wrote:

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...

The outright torque number is uninteresting. The interesting point is the torque curve and where in the rev range torque is available. The kz750-4 sure has good torque, but it is way up in the rev range. 

Unfortunately I haven’t seen many torque graphs for the 750 twin or for that matter the other parallel twins of the era like XS650 and T140.

If you look at the graphs Doc posted, the KZ650's torque starts just above where the 650's  is at,  a couple of hundred revs higher, where  the twin virtually stays the same, the 650's keeps climbing , I would put my money on the 650 being faster everywhere than the 750 Twin, with 100cc less, the 750/4 is faster again.  I'm riding my sons KZ650 every day and it has ample torque, not as much as my old GSXR1100, or any of the other {around 30},  larger bikes I've owned, but plenty for the street , and I don't have to rev the balls out of it to use it either, it would eat those heavy old twins for dinner, the KZ750/4 has more power again. How many of these bikes have you  owned  or ridden ?  

Look at Docs modified figures for the twin below, after all that work, my 1982 GPZ 750 has virtually the same HP stock, with a few tweeks , and a lot less money, i'll be around 100HP..
You could put a set of Cruizin image 810 pistons, around $180 dollars,a second hand set of later GPZ cams and you're instantly blowing the doors of the old twin for very little money. I know what I'd do.....

www.cruzinimage.net/2017/08/25/80-85-kaw...re-pistons-kit-69mm/
 

I am a little surprised it did not show a bigger difference down low, but it is still there.

But you are missing the point with the 750-4 being faster. I am not after "faster". I am very well aware of the fact that the 750-4 is faster in standard form and if I put the same into what I might gain 4-5 hp on a twin, then it will be even faster still. But that is not what I am after. I do a lot of city driving and would love for the bike to work better in low rpm.

Unfortunately I have never tried riding a 750 twin. I have ridden an old Triumph Bonneville 650 and that pulled almost from idle.

wrote:

Did some one say modified KZ750 twin??!!!

Stock KZ750B about 47-50 WHP
light port and polish, RS34 carbs, digital ignition, full exhaust, bored 1mm over with close to stock compression - 55 WHP
light port and polish, RS34 carbs, digital ignition, full exhaust, high comp pistons (stock bore), custom cams - 62 WHP
currently working on a big bore engine with oversized titantium valves - 906cc hoping for 80-85 WHP

I don't remember the torque because that was never my focus, these bikes have plenty of torque, but lack mid to top end power IMO. The factory dyno numbers are not accurate but they are a good apples-to-apples comparision.

 

Thanks a lot for the input and dyno charts. That is very interesting. I was expecting a little bigger difference down low, but there is no question that it is there!

Also nice to see some data points on what to expect if modifying the old twins. If you ever get to 80-85 whp, then that must be quite the ride

wrote:

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?

Stick some VM26's on it from a Z900/Z1000 ( 24's too small and 28's harder to find as everyone wants them ) .
Same spacing on all z650,750,900,1000 aircooled fours..
Use the 900/1000 inlet rubbers and slightly elongate the two holes on them equally inwards to fit the closer spaced mounting holes on the head. 
Unless you plan on rigging up some tubes to the airbox you'll need air correctors and some quality pod filters and it'll go well.
 

wrote:

Stick some VM26's on it from a Z900/Z1000 ( 24's too small and 28's harder to find as everyone wants them ) .
Same spacing on all z650,750,900,1000 aircooled fours..
Use the 900/1000 inlet rubbers and slightly elongate the two holes on them equally inwards to fit the closer spaced mounting holes on the head. 
Unless you plan on rigging up some tubes to the airbox you'll need air correctors and some quality pod filters and it'll go well.

 

VM26 carbs are probably not what OP is after - they are a good 'upgrade' if you want top end power as they will not have the restrictions that the cv carbs have. A 26mm slide carb cannot be compared to a 26mm CV carb. As noted here, the VM26 came stock on the 900s and 1000s - they will work on a 750 but you probably won't see any real gains unless you're combining with head and exhaust work that needs extra flow at high rpm.

I can't say for certain (I've never done it) but I would expect that putting a slide carb from a larger engine onto a smaller engine that had CV carbs would just kill the low-mid torque for the sake of high rpm performance, exactly what OP wants to avoid.