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!
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:
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.
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:
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.
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.
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.
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.
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.
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!