Carburetor Size Selection
When it comes to selecting an OKO carburetor for your motorcycle, picking the right size is the most important factor in getting your machine to run right at all throttle settings.  The metering of the fuel/air mixture of the K series of OKOs is very precise, but depends on the correct amount of vacuum across the nozzle/needle.  You can adjust the pilot or slow jet circuit and the main jet circuit to work correctly with more than one size carburetor, but the key to optimum performance is selecting the size carburetor that transitions the metering of the fuel mixture from the pilot circuit to the main circuit at the precise "time".  At small throttle openings, the pilot circuit controls the fuel metering.  As the throttle is opened more, the pilot circuit becomes "maxed out", only being able to provide so much fuel to keep up with the increased amount of air passing through the bore of the carburetor.  At this point it is imperative that fuel begins to flow up through the needle and nozzle to meet the demands of the increased air flow.  The "right size" OKO will begin fuel flow from the nozzle just as the pilot circuit is at its maximum capacity, thereby assuring the correct mixture of fuel to air is maintained.  If the OKO  K series carburetor is too small, fuel will begin flowing from the nozzle too early due to the higher vacuum created by the small bore.  This will cause there to be too much fuel at the transition area and cause a "rich spot" at about 1/8 to 1/4 throttle.  Conversely, if the OKO carburetor fitted is too big, fuel will not begin to flow through the nozzle soon enough due to less vacuum created by the larger bore.  The result will be a lean spot between 1/8 to 1/4 throttle.  When dialing in the OKO for the first time on a bike we are not familiar with, this is how we determine the correct size, and we offer this information to help others determine which size OKO is right for their machine.  Note:  We now offer 21mm through 26mm carburetors with a "Stretched" bore to give more top end than the stock carburetors.  These are special order items, but we just need an extra day to modify them before shipping.  Check our "Sales" page for the "Ultimate Performer". 
We recommend that you first dial in the pilot circuit on your OKO K-100 carburetor.  The "sweet spot" with the air screw is where you have the highest idle (of course you are turning the idle speed screw down as you get closer to that spot).  This "best idle" spot may encompass a quarter to half a turn in or out of the air screw.  Within  that area, find the spot where/when you slightly blip the throttle, the engine accelerates cleanly and smoothly, and quickly returns to idle without surging.  Surging and slow return to idle indicate leanness and the cure is to turn "in" the air screw to lessen the amount of air.  If the engine four-strokes when the throttle is blipped (fires every other time), that indicates richness and the cure is to turn the air screw "out" to increase the amount of air.  Once this spot is found you can move on to higher throttle openings.  If you try higher throttle openings before you have the idle set correctly, you will not be able to properly tune these areas due to "carry-over" of the idle circuit.  It is also very important to recheck the idle adjustment after all else is tuned, when the ingine is hot.  This will ensure the proper setting.

For more tunng information, especially regarding needle settings and main jet selection, please visit our tuning page.
Latest Developments
It appears there are two trains of thought when using the OKO K-101 carburetors.  There are some who market the OKOs who believe that the larger sizes are the way to go, giving optimum top end.  For trials, it is thought that the weaker bottom end provided by the larger carburetors actually helps the tires to hook up better.  To a degree, I understand this philosophy, especially for top riders, who need as much power as they can get while attempting some of the more difficult, large obstacles, like rock ledges and other tall, vertical surfaces.  These riders are expert enough to not need bottom end, often relying on clutching technique with more throttle to give traction when necessary.  So I certainly agree, that for some of the expert riders, the larger carburetors give them more of what they need.

At Mid-Atlantic Trials, we have perhaps the opposite approach to fitting the right sized carburetor, and have found this works with road bikes as well as trials bikes, or for any other form of riding where low and mid-range power is important.  We fit carburetors to various bikes for the person who needs a smooth transition of power across the throttle range.  What we have found, is that many sizes of OKO K-101 carburetors will work on any given engine size.  A 28mm or even a 30mm will work on a Yamaha TY 175.  The sacrifice with the larger carb is less power at smaller throttle openings. So when we put together a kit for a given motorcycle, we base the size of the carburetor on which one gives a smooth transition of power at all throttle ranges, from idle through full throttle.  This may mean a small sacrifice in top end power when compared to the larger sizes, but we have found that most riders, especially those who ride older bikes, and mostly for fun, do not need all the extra top end power a larger carburetor provides.  The average rider is looking for the motorcycle to run smoothly and cleanly at all throttle openings.  With this in mind, we continue to test various sizes of K-101s on all the bikes we fit them to, in an effort to find which size works best overall. 
For bikes other than road or trials bikes, typically cross-country and motocross bikes, we now have riders experimenting with different sizes of OKOs They are fitting a smaller carb for tighter events, and larger carbs for events that have more straight, full throttle sections.   So the experimenting goes on, and we will keep you abreast of what we learn as we learn it. 

We will now be offering our kits in various sizes.  All will contain the same adapters and hardware, but you will be able to choose a size other than what we offer in our standard kit.  What this will mean from a guarantee aspect is that we will still offer the 30 day guarantee on the standard size kit, meaning you can return it if you can't dial it in satisfactorily, but we will not be able to offer the same "dial in" guarantee on the optional sizes.

You will also notice a change in some of the standard sizes, as we find a different size than previously believed, gives better performance.  Examples are the Montesa Cota 348, which will now come as a 26mm.  We have had a few returns due to a leanness just off of idle with the 28mm, and the 26 has proved to be just right.  We have also re-evaluated the size for the Yamaha TY 175.  Latest tests show the 21mm to give the best performance over the 24mm and 26mm.

Lately, we are finding quite a few bikes that are giving the best performance with the needle clip between one of the notches.  Some of our kits will now come with the needle shimmed.  For those that are not shimmed, we will provide a washer/shim in case fine adjustment calls for it.  We will also be providing a shim with each needle purchased.  They will also be available as a set of five shims.

We have started to open up the hole for the drain plug at the bottom of the carburetor to allow the removal/replacement of the slow jet without removing the float bowl.  As the bowl comes from the factory, removal of the bottom drain plug allows you to remove and replace the main jet, but it is a little small for access to the slow jet.  However, using a chain saw file, we have enlarged the notch in the threads on the bottom of the bowl, which now provides changing of the slow jet as well without removal of the float bowl.  We have placed photos on the tuning page for you.

Severalyears ago we received a batch of carburetors that had poor placement of the jet block gasket.  Symptoms of poor sealing of this gasket are leanness, especially at idle, which require much larger slow jets to counteract, and sometimes fuel also leaks through and richens the mixture, especially when the bike is wheelied or jumped. 
Note:  Finally, the factory has replaced the old jet block gasket with a new fatter one that is a nice fit and provides a good seal without having to carefully place it on the jet block.  If you have a thinner one, we now offer the new, thick one in our gasket kit.

We do not have the opportuntiy to extensively test every model we fit a carburetor to.  We rely on feedback from fellow riders to help understand the range of jetting that works best.  To really test jetting, a bike needs to be ridden for a good length of time and under a variety of conditions.  We test main jets under hard acceleration, especially making sure they are not too lean.  You may find that your installed main jet, when tested over time, is too large.  If we have not provided the proper main jet, just contact us and we will exchange it.

Concerning the K-104 sizes from 32mm to 40mm, we have run into at least one instance where no matter what jetting we used we could not get rid of a lean spot just off of idle when all else dialed in nicely.  Ultimately we took a chain saw file and put a very shallow (less than a mm) notch in the bottom center of the slide.  This acted as a change in the slide cutaway which richened the mixture just above idle.  It also richened the idle a little that required changing to a slightly smaller pilot jet.

I have previously heard that the PWKs suffer from the float bowls flooding.  I have not had any issues with genuine OKOs, but recently had the opportunity to work on a bike with a copy that was flooding.  This OKO copy would flood when the float bowl became nearly empty and was then refilled.  It turned out that the float needle would cock slightly sideways when the bowl was low on fuel and therefore "stick" open and consequently overflow. 

Bigger is Not Always Better
With all of the information available about selecting carburetor size, we still often encounter those who believe "bigger is always better".  Often when switching from a round slide carburetor to a D slide OKO we have found that a size smaller is the best size.  Though bigger carburetors will give more top end performance, it is always at the cost of losing bottom end and mid-range performance.  Unless you use your bike for racing and need top end more than anything, most likely a smaller carburetor will give better power at smaller throttle openings.  Most riders use more throttle  between one quarter and three quarters than wide open, and this is where a smaller carburetor will outperform a larger one every time.  As an old schooler, I remember how Gary Nixon won Daytona back in the day using 24mm carburetors on his 500cc Triumph.   You want the size that will give you the best overall performance and not the one that just gives the best top end.