Servicing and Finding Fault in CV Carburetors

For the past few years we are getting CV (Constant Velocity) Carburetors in our motorcycles which are giving us very good performance and efficiency. Earlier the carbs were having round cylinder slide for throttling the engine which was directly connected to throttle cable. In CV carb. there are two throttles bodies, one is butterfly and another is round vacuum assisted cylinder. Butterfly slide connects directly to accelerator wire but round slide works on air and vacuum.

stock-cv-carbCV carbs. require special touch and understanding the construction. Earlier, mechanics used to clean carbs with some sharp needle and gasoline. But that used to damaged main, pilot jets and other tracks. By using sharp needle jet sizes used to get change. Which was not visible by regular eye and so it used to change the efficiency and performance.

Now there are some cleaning liquids available in the market which can dissolve the carbon layer very fast and efficiently. Which also keeps the jets and carb body material intact. In CV carb. the most and delicate part is diaphragm, which can get damaged easily.

Generally, carburetors are not cleaned often nowadays because of clean fuel. But sometimes we need to clean it due to tank rust or foreign material entry from intake side. So try to open till float chamber, where you can see main jet and pilot jet, use spray for cleaning and compressed air and re-assemble the carb properly with equal tightening.

After cleaning and fixing the carb. put back to the intake manifold and start the engine. Set the air screw at 2.5 to 3 full turn or 5 to 6 half turn and set the RPM as per company slandered. But make sure while setting RPM engine is warm. Because cold engines requires little high RPM which shows which gives wrong reading at high temperature.

The exhaust should give you moisture feel or may be some water droplets. If you feel the moisture or water droplets that means you engine is properly tuned. Sometimes the carb float chamber gives cold feel or you can even see moisture on the float chamber.

In some cases rider comes with acceleration problem. Check whether air screw is loose or not, because generally in CV carb. air screw gets jammed by oxidation deposition so always try to clean the air screw when ever you go for servicing.  Check the diaphragm in light for small pin holes, if u see any hole then replace it. Then check the diaphragm whether it is working or not by blowing some amount of air in given holes. if the diaphragm is working then carb should be fine, but in this case you must check the diaphragm from both end of carb.

First end is inlet manifold and second end is air filter end. Air filter end has got two holes on the top of carb. periphery, blow some air then the slide will activate. Next try to blow air from first end. The air should only activate slider, should not get leaked from any other side. If the air is entering through the slide or needle portion that means the carburetor body is increased by some microns.

The ID of carb body which is provided for slider if it increases then the motorcycle will never go beyond 1200 to 1500 RPM or it may not stay at idle either, the bike will get heat and acceleration will slower down after some time. In that case carb. body or full carb. has to be changed, check and handle the carb very carefully and clean it from any of your skilled mechanic.

All the given information is for understanding the CV carb and for maintenance, If you are a mechanic then handle the carb very carefully. But if you are not a mechanic then try not to open the carburetor alone. Make sure you are under observation of any skilled mechanic. Otherwise you may damage the carburetor. CV carburetor is one of the costly spare so you might get fooled or ask your mechanic to handle it very carefully. So keep your eyes open while your mechanic is working on it.

Always use proper branded tools and sprays for repair. Your machine also need care just like our human body. Remember both the bodies are delicate.

For more details about high performance and other technical things about bikes you can call us on +91 98222 12496 or you can mail us at

– Chinmay Dangre