Give’em Some Gasoline: Installing CV Carbs on Cossack Motorcycles

a girl at a gas station

Hi all! Today I want to share with you a simple way to increase horsepower of your beloved cossack motorcycle! All you have to do is to replace standard carburetors with constant vacuum carburetors. Yeap, it’s that simple!

Why Installing CV Carbs on Your Cossack Motorcycle

Constant vacuum carburetors (also known as constant velocity, CV and constant depression carbs) are the most sophisticated carburetors people managed to create so far and installing such carbs on your motorcycle will bring lots of benefits:

  • Increased horsepower
  • Improved engine performance
  • Fuel economy
  • Smooth engine operation

Only electronic fuel injection can be better than CV carbs. BTW, some people even say that they prefer sport bikes with CV carbs to the ones with fuel injection systems, but that’s another story. Anyway, the advantages of such tuning (I mean installing CV carbs) are definitely worth spending some efforts on doing this. Let’s see how this can be done on different cossack bikes.

Installing Constant Vacuum Carburetors for Dnipro MT, Ural and K-750

Even though replacing standard carbs with constant vacuum ones sounds like a piece of cake, in fact it can be hard to find the right carbs and install them on a Dnipro, Ural or K-750 engine, especially for people who haven’t dealt with such things before. Generally, I can think of three options here:

Option #1: Buy a set of carburetors from a new Ural (Wolf, Solo)

Some late models of Ural motorcycles, at least those that were to be imported to the US and thus had meet EPA requirements, were equipped with Keihin CVK32 (before the EFI system has been implemented). That’s why you can buy a set of these new carbs from a not too old Ural model and install it on your Ural, Dnepr MT or K-750. I’m not sure, but I guess you will be able to find such sets on Ebay or at some local motorcycle stores.

Option #2: Buy a set of ready-to-use CV carbs from some motorcycle guru

I don’t know if this option is available in your country, but here in Ukraine we have several motorcycle specialists who buy used CV carbs from different motorcycles (mainly Japanese ones), fix them, if it’s needed, create special adapters for these carbs to be mounted on Dnepr and Ural engines, make all necessary adjustments (e.g. replacing jets, etc.) and then sell these “plug and play ” (or it would be better to say “install and ride”) sets. Personally I recommend this option to all my friends with Dnipro MT bikes, because this doesn’t require any special knowledge and it takes only a few minutes to install such carbs. And one more advantage of this option is the price. A pair of these ready-to-use CV carbs cost about $100. BTW, if you would like to buy such a set, hit me up and I might be able to help you with this.

Option #3: Buy two used CV carburetors and adjust them yourself.

This is the cheapest option, but it requires some knowledge and ability to craft different things. There is a chance to find used CV carbs for $50 or even less. Of course, used carbs might require replacement of some parts (membranes, needles, jets) because of them being worn. You never know how much it will cost: a few bucks or $50.

Once you restore the working condition of the carbs, you will need to mount them on your engine. Please take a look at the photos to see how suitable adapters look like. They can be made of small tubes and metal plates with 2 small and 1 big holes using a welding machine. Once the adapters are ready you can connect them with the carbs via rubber tubes. You will also need to connect throttle cables to the carbs. Check the photos I provided to see how it can be done.

Now, when you mounted the carbs on your engine, you need to adjust them. If you have Keihin CVK32 carbs or similar ones (such carbs used to be installed on some Urals, Kawasaki Vulcan 500 motorcycles, etc.), you won’t need to spend time finding the right jets and can make only minor adjustments via needle clip positions and mixture screws (they can be found at the bottom of your carbs). If you have some other CV carbs, the chances are that you will need to try several different jets before you find the right ones for your engine.

The third option is obviously the cheapest, but it can be a real pain in the butt, especially if you do this for the first time. That’s why my personal choice is the second option. Yet, if you don’t like installing used parts or don’t know where to buy a set of read-to-use carbs (I can help you with this), than the only option that is left for you is buying a pair of new carbs from some late Ural model.

Installing a Constant Vacuum Carburetor on Jawa 350 (type 634)

If you are a proud owner of a legendary Jawa 350 (type 634), the good news for you is that you can also install a CV carb on your motorcycle. A friend of mine did this last summer, here is his experience.

The ideal carb for this case will be Mikuni VM-26. There might be other constant vacuum carbs (produced by Keihin, Bing, etc.) that are suitable for two-stroke engines, but I don’t know about them. As for Mikuni VM-26, it can work on both two- and four-stroke engines and its tech characteristics are close-to-ideal in case of this motorcycle.

The first thing you will need to mount this carb is the adapter to connect it to the engine. Fortunately, such adapters exist and can be bought. The adapter I’m talking about is actually for mounting a carburetor from a Jawa 350 (type 638) on a Jawa 350 (type 634), but it meets our needs perfectly.

Once you buy such an adapter (or craft your own one), you will need to slightly shorten the air duct rubber tube, because your new carb is a bit bigger than a standard one.

Last but not least, you need to replace the main jet of your new CV carb. The thing is that a standard main jet of the Mikuni VM26 (it’s marked “190”) is a bit too much for your Jawa motorcycle, i.e. it will work nice, but when you open the throttle valve at 70% or more, the amount of the air/fuel mixture provided will be too much for the engine. After trying different jets, my friend found the right one – the one marked “130”. Now, when you have everything needed, all you need is replace the main jet, mount the carb on your bike and go have some fun on a road. You’ll love the way your bike behaves with the new carb, I promise :).

Just to sum up, if you want your Jawa 350 (type 634) have better performance and make it have more power, installing the Mikuni VM-26 carb with the main jet “130” will be a wise decision.

Installing a CV carb on IZH Planeta and Jupiter

I don’t have personal experience in installing CV carbs on IZH motorcycles, so I can only theorize about this topic. There was a period when IZH Planeta Sport (PS) motorcycles were equipped with Mikuni carbs, but I’m not sure what marking those carbs had and whether they are still available on the market. All IZH motorcycles have 2-stroke engines and the only CV carbs for 2-stroke engines I know about are Mikuni VM series carburetors. So, I believe the best option will be to get a carb from this series and spend some time on trying different jets to find the right one. In case of Jawa 634, which has a 26mm diffuser, I suggested getting a Mikuni VM26 carb. IZH motorcycles have different diffusers, that’s why you should consider this fact when looking for the right carb.

  • IZH Planeta 3, 4, 5 & Planeta Sport – 32mm diffuser – Mikuni VM32-33 and Mikuni VM32-193 (both carbs are almost identical)
  • IZH Jupiter 1, 2 – 24mm diffuser – Mikuni VM24
  • IZH Jupiter 3 – 28mm diffuser – Mikuni VM30
  • IZH Jupiter 4 – 30mm diffuser – Mikuni VM30

Please note that the diameters of the diffusers might not be 100% correct, so you should double check this info to be absolutely sure regarding the needed carb. Speaking of the carbs, it’s not a big deal if you use, for example, a Mikuni VM32 for IZH Jupiter 4, which has a 30mm diffuser, it will even make the engine more powerful at high rpm, but you’ll slightly lose performance at low rpm.

Anyway, once you get the right carb, you will need to try riding your motorcycle with different main jets and decide which one suits your engine better.

If I ever get some related info, I will update the article and if you have some questions or need some additional explanations, please feel free to leave your comment in the section below.

Meanwhile, enjoy every moment you can spend riding your bike, because that’s the freedom we are all looking for!

P.S. If you find this article useful, please share it via social networks for other bikers to find it!

Alex White
Alex White
I'm just a guy who values 3 things in life: girls, bikes and freedom. I get them all with my custom-built Dnipro MT. Cossack motorcycles are truly unique and reliable bikes if being treated right, and I decided to create this blog and share my vast experience and love for cossack bikes with other bikers all over the world. Need my help or advice? Contact me via skype or email.



    Hi this site is very helpful I live in England and have 1966 kmz k750 /denpr it is in very rough condition bought off e/bay and when it turn up was in worst condition than photos so going to be long project it has k302 carbs I am interasted in option 2 Keihin cvk32 the complete kit just fit and ride and where to buy this kit, the carbs I have I hear they are not very good as money is tight at the moment divorce / seetlement would the k68pekar be any good.
    Rob .

    • Alex White says:

      Hey Robert,

      Congrats on your purchase. It’s sad it turned out to be in worse condition than you expected. Well, sh*t happens 😉 Don’t worry, consider it as a chance to know your bike better and to make it be taken care properly. Buying a used buy that looks good at first and then gets broken during some long ride would have been much worse.

      You are right about the original carbs – it doesn’t matter what condition they are, they are “morally old” and highly inefficient. As for K-68, I had those on my bike, they worked well, at least I thought so. But when I replaced them with CV ones I understood how horrible the K-68 were 🙂 I mean CV carbs are WAY better than K-6X.

      I’ll call one local guy who deals with carbs and let you know if he has a set of CV carbs and how much he wants for them.

  2. Alex N says:

    Hey Alex,

    Are you sure Mikuni VM26 is a CV?
    Can you point out where the diaphragm is?


    On the positive side – yes they are good match for 634, and yes 130 main is the right number. Also, move the needle to the lowest position (top notch)

    P.S. girls, bikes and freedom – hell yeah, don’t forget about rock-n-roll !

    • Alex White says:

      Hey Alex,

      Thank you for bringing up this point! If I’m not mistaken, the last time I held it in my hands, it had 2 “dampers” (I’m not sure if that’s the correct English term in this case), one of which was connected to a throttle cable and another one, the one connected to the needle, had a spring pushing it down (to the “closed” position).

      I took a few minutes to google this matter, just to be sure everything is correct and here is the photo I found this one –

      This makes me think that Mikuni VM26 is a CV. Yet, I might have found an incorrect photo. I don’t own an Jawa motorcycle and thus don’t have this carb to check it and give you 100% accurate answer. Sorry about that 😛

      P.S. Yeah, my bad, girls, bikes, freedom and ROCK-N-ROLL 🙂

  3. Phil Whitehead says:

    Alex , the VM26 is NOT a CV carb . A CV carb has the throttle slide lifted by negative pressure above a diaphragm not by the throttle cable directly . The VM26 is still a good carb just not a CV carb ,

    Regards ,


  4. paolo says:

    ciao buongiornosulla la mia dnepr mt 11 ho messo due carburatori k68 ma quando vado in salita non funziona bene e poco dopo inizia a scoppiettare e si ferma cosa puo essere? in pianura gira bene..

    • Alex White says:

      Hey Paolo,

      If I got your message right, you’re saying that your carbs work fine, but when you climb some hill, you start experiencing issues with them. I believe you should check your level of fuel in the carbs. I can’t think of any other reason for your carbs to act different when on a plain and on a hill.

Leave a Reply

Your email address will not be published. Required fields are marked *