KZ750 oversize intake valves?

Is that a thing? And if so, where can you buy larger valves for these motors?

I'm about to send my 1980 KZ750e head off for some work (alongside turning it into an 810), so it's a good opportunity to explore this.

APE say they don't do KZ750 valves... they only seem to start from the 900 and up... anyone got knowledge on what makes those valves different/unsuitable for the 750 motors?

Here is a nice writeup about modding the 750 engine... members.tripod.com/gpz_rider/technical/FAQ2000.htm

I'm not sure about adding larger valves. Sounds expensive.

There is/or was since I have a set of Manley 1mm over size.
KZ900/1000 valves are the same except for the intake valve head diameter.
The KZ750 is 1mm smaller. Exhaust is the same size.
The question is do you really needed it?
Have you done the math for bore size vs valve size, intake to exhaust ratio?


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Yeah good questions.
I see so much when searching here on this topic for the 900's and 1000's, but nothing for the 750 blocks.

My train of thought is Inamura-san would have designed the 34mm intake valves as optimal for a 750, so if we go to an 810cc, going 35mm (or more?) would make sense.

I can't admit to being across how to do the math on all this though. Was secretly hoping someone here has done the thinking over the past 40 years of these bikes!

When going from the 650 to 750, Kawasaki enlarged both the intake by 1mm and exhaust valve by 2mm. ( I think those are correct numbers. But irregardless, they are bigger)
That put the Intake valve on the very edge of the combustion chamber. Recutting the valve seat (if there is even room left to do so) for the 1mm oversize valve would encroach into the quench ring around the combustion chamber. Even decking a kz750 head a very small amount will cut into the intake valve seat.
The second issue I believe is that there is a issue of valves touching during the exhaust cycle as both intake and exhaust valves are open at the same time. That extra .5mm may lead to the intake and exhaust valve trying to occupy the same space at the same time. Especially at high RPM’s.
On the Kz1000 head however, there is ample space for larger valves and plenty of room for them to work in. Google image search both a 750 and 1000 head and you will notice the difference in combustion chamber shapes and size.

What are the 650 and 750 valve sizes? I suspect that they are already too big for the 750, and possibly the exhaust valve is too big in relation to the intake, so going to a larger displacement will make the valve size more proportional for street use. Here’s an excerpt from “Four stroke performance tuning”

Look up Schumann Motor Works for all your custom valve train needs....

Daftrusty wrote: When going from the 650 to 750, Kawasaki enlarged both the intake by 1mm and exhaust valve by 2mm. ( I think those are correct numbers. But irregardless, they are bigger)
That put the Intake valve on the very edge of the combustion chamber. Recutting the valve seat (if there is even room left to do so) for the 1mm oversize valve would encroach into the quench ring around the combustion chamber. Even decking a kz750 head a very small amount will cut into the intake valve seat.

That makes a lot of sense. I have a 750 head sitting out in the open at the moment, so to your point, I'll have a better look at it.

DoctoRot wrote: What are the 650 and 750 valve sizes? I suspect that they are already too big for the 750, and possibly the exhaust valve is too big in relation to the intake, so going to a larger displacement will make the valve size more proportional for street use. Here’s an excerpt from “Four stroke performance tuning”

Amazing!!! Gee I love this forum for exactly this kind of knowledge sharing!!
I ended up finding an online valve size calculator , and you're seemingly correct in your thinking. The calculator suggested 33.5mm intake and 28.5mm exhaust valves for an 810cc motor (Bore 69mm, Stroke 54mm, Max rpm 10k).

Factory sizes: KZ650 | KZ750
Intake Valve: 33mm | 34mm
Exhaust Valve: 28mm | 30mm

Makes you think, based on that last screenshot from DoctoRot, an 810 kit might be best using a 650 head for its 30mm exhaust valves, but then with oversized 34mm intake valves from the 750 head.

In theory using the 650 head with a 750 intake valve works best for a 810 kit. however these are just theories and real world results can differ. It also depends on your application, if you want a torquey street machine then the smaller valves will be better, if you want a screaming race weapon the bigger valves are probably preferable. However valves are only one part of the equation. port size, volume, and cam will play a roll as well. better flowing carbs will help big time. I suspect that like many Japanese manufactures of this time period they ended up looking for a high overall HP number for marketing purposes rather than real world driveablilty. think GSXR 1100 vs bandit 1200.

What I found when I started developing the big bore kit for my kz750 twin was that the stock valves were oversized for 750cc, and a poor ratio (too big exhaust), the ports were too big, but a good volume. By adding 150cc displacement, and 1mm bigger on the intake valve. Now the valve sizes are more ideal. I kept the same port sizes but quite a bit of porting was necessary to get the port volume corrected.

Well now you're really making me think!
And I probably bought that 750 head for no reason now! ...other than for the intake valves maybe

This is all with RS34 flatslides, GPZ cams, and briefing in a port & polish job, with a light milling too.

What is the intake port size of the stock 650 and 750 head?