Selecting the Correct Size OKO
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 its best at all throttle openings. The metering of the fuel/air mixture of the K series OKOs is very precise, but depends on the correct amount of vacuum through the bore. You can adjust the pilot or slow jet circuit with the air screw or pilot jet size, the needle via its clip placement and the main jet via its size on carburetors that are too large or small, but the key is getting the idle circuit to transition to the needle circuit just as the idle circuit has maxed out. This should happen at about 1/8th throttle. If the carburetor is too small, the vacuum through the bore will initiate the needle circuit too soon and cause a rich spot there. If the carburetor is too large, the needle circuit will not take over soon enough, resulting in a lean spot there, just off of idle. Also, when the throttle is hit hard there will be hesitation or bogging if the size isn't right. The correct size carburetor will initiate the needle circuit just as the idle circuit maxes out, providing a smooth transition off of the idle circuit.
Unfortunately, there are still riders who believe that bigger is always better. Bigger will give you more top end, but you will sacrifice low and mid-range power. While round slide carburetors use different slide cutaways to help rectify this situation to some degree, OKO carburetors' slides do not have different cutaways, and so depend on the bore size to regulate the vacuum through the bore. Because PWK or D-slide carburetors atomize fuel better than round slides, often we have found that a size smaller OKO will be the correct replacement for a round slide. An example of this and what brought me to appreciate OKOs, is when I replaced a 28mm Bing carburetor on my 326cc Bultaco Sherpa T with an OKO. The 26mm OKO gave me more power and better throttle response than the Bing without sacrificing top end. Actually when riding through the woods with the Bing carburetor I was always switching back and forth between third and fourth gears when going up hill or at lower rpms. With the OKO I was able to get into fourth gear and stay there, power being sufficient to pull well from lower rpms and while climbing. Our kits use the selected size we send due to our testing to find the best size. It is often a size smaller than the carburetor we replaced, but is the size that works best at all throttle openings.
