Can I do straight carb swap with KZ400 carb?


XS400 Member
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Cape Town, South Africa
Hi, I am new to this Forum.

I would like to know if KZ400 carbs will be a straight fit (intake to intake measurement)
KZ400 is also 32. I am having problems where I get foulded spark plugs and I just want to know if it will fit my XS. I can get one (swapping for a tool set) but I dont want to swap if its going to be useless.

KZ400 carb is a Keihin VC32

My XS400 is a 1982 with the Mikuni BS32

Thank you guys
Welcome to the forum!

I don't know about the Keihins. Can you have the person that has them take some measurements of the diameter of the spigots and their center to center measurement to see if it is even close? The only carbs that I think might be close to drop-in for the XS400 SOHC bikes is a set of GS500 carbs (Help...anyone?! Carb identification & general chaos...), but even there, I am not totally sure about it.
Thank you for the reply Cpt. Chrome.

My XS is not with me at the moment. Can you please tell me what the measurement is center to center of our XS400s then i can compare with the KZ400 carbs.

Will appreciate it.
I just want to confirm some important details with you. You have a 1982 bike and at least here in the US in that year there were two different XS400s sold – the Single Overhead Cam (SOHC) and the Dual Overhead Cam (DOHC, sometimes known by the name Seca or Maxim). Because I don’t know what models managed to get imported into South Africa or brought over later, I am going to describe both.

The good news – both SOHC and DOHC bikes used Mikuni BS34 carbs (you said you have BS32, but I think that must be a typo) they just had different internal jetting/metering. The bad news – the ganged up configuration of the DOHC carbs is closer together than the SOHC carbs:


I have the SOHC model and the center-to-center spacing of the carbs is 110mm. I don’t know the spacing for the DOHC carbs. Regardless, be careful to know which bike you have.

I am going to add one thing, and I don’t mean to piss in your beer by saying it as I don’t know any details about the exact issue and your history of trying to fix it. But typically it is a lot easier to fix the stock BS34 carbs than to fit something different (that is also 40+ years old), unless you have a bunch of experience tuning jets and making your own small (or large) changes to the throttle linkage bits. I absolutely think it can be done, but Mikuni and Yamaha did a bunch of things with the stock carb they put on your bike that are hard to change, like the pressed-in jets and the shape of the needle, that starting with stock carbs is faster and simpler, in my opinion. Now go drink that beer. The amber color of the fluid is not from me – Cheers!
Hey howsit. Thanks so much. Its SOHC. I will give it a try and see what comes of it. The throttle linkage of KZ400 is in middle so ill just move cable to middle between the carbs.

If all fails, ive tried and then ill move on to working on XS cabs.

Any ideas as to why spark plags get alot of carbon on very quick?

Will carck that beer in a bit :)
Chiao chiao

Hi again. Here are the KZ400 carbs. Also 110mm and check the throttle mount position, ill just put cable in middle.

I dont see why this carb should not work as they both same ie. 400cc twin..

What are your thoughts?
Thanks again.


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Here are the KZ400 carbs. Also 110mm and check the throttle mount position, I'll just put cable in middle.
That looks about as close as it comes to finding a ganged up pair of carbs that will fit. It looks like the spigot diameter is 35mm or so and hopefully that is close to the carb holders on the XS. You theoretically should be able to get this to work, but some tuning might be necessary as the KZ is a nearly square engine (bore is pretty much equal to stroke) whereas the XS is over square (bore is significantly larger than stroke). But if you are so inclined, try them and let us know how it goes as I don't think I have seen anyone do this.

Here are the plugs, do you know what is causing the fouling?

The carbs are creating too rich of a mixture. The idle mixture screw might be too far out, or the jets too large, or the needle position too high or any combination of those. One of the plugs looks wet as well, and if that is oil, then you might have some bad rings in that cylinder or a valve seal leak.
If it were me, I would first perform a compression test of the cylinders. That will tell you if the rings or the valves are an issue.
If compression is low ==> pull the top end of the engine apart and install new rings, clean and lap the valves and install new valve oil seals.

If compression was okay I would clean the plugs or install new ones and run them to see if they foul.
If fouling and wet with oil ==> pull the head and replace the valve oil seals (you might as well lap the valves too as you are already there).
If fouling and you have dry carbon and soot ==> clean the BS34 carbs completely and tune them (set float level, set idle mix screws and adjust the pilot and main jets) to get the fuel mixture in the cylinders right.
Hi, thanks for all the info. Will check all what you have mentioned soon as I get the bike back.

If it is rings, will she then smoke out exhausts? Because there no abnormal smoke..