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.
CV 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 corepower@rediff.com
- Chinmay Dangre
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{ 19 comments… read them below or add one }
Another super article Chinmay. Though its the age of the FI bikes, still carbs will be mass produced.
Thank You MotorBeam.com, In India the carburetor segment will not go off, Because though ECU is easy and best but understanding is hard…..
Superb article Chinmay. In India most of the 135 and above new generation bikes are equipped with CV Carb. This will continue and ECU will not replace CV carb in near future. I have a problem with my Honda Unicorn carb and even Honda authorized service centre people not ready to open the carburetor. Do you know any good mechanic in Bangalore for servicing the CV carb?
Well Jayan I am sorry but I am from Pune. So don’t know anything about Bangalore city. Besides if you let me know what kind of problem you are facing then I might able to guide you in repairs.
Generally CV carb has got only two problem which are related to diaphragm. Which is very hard to diagnose. Other things like Idle circuit or High RPM circuit is easy to understand. In diaphragm check for pin holes with dirt free clean hands, keep all sharp objects away from diaphragm while handling it. If there is no hole the mostly the diameter of the carburetor benjo body got increased or the diameter of round slide got decreased. This all happend because of either manufacturing problem or dirt problem, Because there is a friction in that when it works. So you will have to change mostly one of the spare either carb. body or slide or may be both. If the spares are not avail. from company then change the carb. But make sure you are not in warranty period. If yes don’t tinker the carb by your own. Get all work done from skilled mechanic.
Still I want to know the problem. And try to record the sound of your bike at Idle and while reving up & send it to me at macasp@gmail.com
Thanks.
Dear Chinmay.
Im facing certain problems with my Honda Unicorn 2005 model.
of late the bike just stops all of a sudden. after continuously kicking it feels as if there is no petrol reaching the engine. After some time it starts but then once i put it into gear and accelerate it just stops. basically the vehicle remains on in idling condition but moment i accelerate it shuts down. 2 -3 times i tried by just letting out some petrol fom the carbeurator by loosening the screw allowing a little bit of petrol to flow out fdrom the bottom of the carb. after doing that the vehicle starts running properly.
Also of late there is a problem in the pick up of late i have felt a sort of loss of power. Sometimes as im riding and i accelerate a bit suddenly the the vehicle just pulls back moment i release the accelerator the vehicle starts moving normally.
Hi Chinmay,
Need your help from you on Honda Unicorn Carburetor.
I have purchased honda unicorn on 20th Nov-04 and I have run 21500 till date.
Having some starting problem & also it gets stopped after going few meters.
I went for servicing and they told that AIR Screw is jammed and it cannot be opened so I need to replace my Carburetor.
Are they correct or is there any other way to open the AIR Screw.
Please advice me.
Thanks & Regards
Anurag
Dear Anurag,
I am sorry for bit late reply. Well generally I found the AF screw gets Jamm in the carb due to oxidation. secondly if the mechanic don’t open it while seriving for removing oxidation then that’s wrong. Use carb spray to clean the carb.
Well it is possible to remove the jammed screw from the body, but chances are very less like in 100%, the chance of removing the jammed screw is hardly 1 or 2% coz the threads get damamged while removing it. The skilled machinist is the only guy who can try to remove. But not others. HE will also say the same thing about the chances. Machinist use drilling to remove the screw or special purpose stod remover kit for that. so it’s one of the difficult and critical job to do. So generally SC guys don’t take that risk and headaches….u know…
STUD remover kit*
informative article.
Dear Chinmay.
Im facing certain problems with my Honda Unicorn 2009 model.
Mostly after I idle my engine say for 20 mins, the bike just starts but does not pickup motion once I apply the 1st gear. It requires that I open the choke and make my vehicle move for atleast 2 meters. this problem is there almost everytime I idle my engine, be it early int he morning or at nites . Basically the vehicle remains on in idling condition but moment i accelerate it shuts down. 2 -3 times i tried by just letting out some petrol fom the carbeurator by loosening the screw allowing a little bit of petrol to flow out fdrom the bottom of the carb. after doing that the vehicle starts running properly.
Also offlate there is a problem in the pick up and i have felt a sort of loss of power. Sometimes as im riding and i accelerate a bit suddenly the the vehicle just pulls back moment i release the accelerator the vehicle starts moving normally.
Hi Chinmay,
went through most of ur articles & found it interesting & informative.well i am also one of the bikers from our country who always have a thirst for more power & speed. I have a Pulsar 200 & i have fitted K&N filter (r-1100). actually i want to upjet my carburetor bcoz i dont feel much difference in performance after installing the K&N.I have inquired & read a lot about upjetting from various websites & magazines but unable to get a very clear picture of it.can u plz help me out like which size jet(and of which bike) i should go for & wht are the tips which can help in improving the performance.
Dear Ballu, I suggest you to check your slow running circuit. Since it’s a BS based fuel system. So check the carb for any vaccum leakage. If all the things are perfect then tune the bike properly. Sometimes wrong tunning can make things worst. Consult with ur mech….Don’t tinker it at home…If you have doubts then you can directly mail me. my id is macasp@gmail.com
Dear Abhishek,
Please feel free to mail me on my above id for detail. Since you have problem but you have not mentioned fitting of K&N in detail..
i m willing to fit this carb in my LB500 .. so can it work .
Dear Rahul,
I really didn’t get your thinking.. If you are unknown to the real fact then let me tell you one thing. You are already running on this type of carb with the size of BS29. So what exactly you want to install? At the most you can go for BS32mm from new DTSI carb version..
Hi chinmay.
I own a bajaj discover 125cc which has cv type carb i have problem whith my bike after i traveled a trip about 600kms which include hill stations for three days at last day my bike had lost pick up and it was not able give good milage at average speed and was getting really hot very soon then i gave to local mechanic he verified the sparks which were turned really black and he suggested me to replace them and then he told me change me the fuel as it may be mixed with water, after this also my bike was haveing same problem then i told him to clean the carb then he noticed the choke the main culprit was choke piston in carb which was blocked full open making more fuel intake after all this i made to banglore and now also i face this problem with pick up and bad milage after i gave bike rest for 8hrs what is the problem what is causing this same problem please help.. I would wish you replay on this before giving to mechanic …thank you and the article was really good..:)
is it possible to install a cv carburettor in yamaha rx-100 if yes then which brand and what other adjustments would have to be done with the engine and will it improve the performance and mileage of the bike
dear chinmay, hyderabad 2/6/11
i have a small problem in my star city 100cc bike is that whenever i slow down from higher speed, i hear explosive noice in the silencer. but if i press the clutch that does not happen. it is embarassing in thre traffic and also the KPl is very low. by opening the plug i found it is dark black indicating rich mixture. i took to the local mechanic but he could not do it instead hs said that it is commom in tvs bikes. my bike is fitted with spaco carb. can u explain how to over come this problem and also the tuning procedure of the carb. i am an elect engineer and have some knowledge about bikes. please reply soon. thank u
On fine morning i was unable to start my Honda Unicorn.After it started then wheneveri accelarte its gets stopped.
So i went to Honda Unicorn servicing center.Mechanic told that Air Screw is jammed either you change carburator or we can open the Air screw at the lathe machine.
My concerns is if they open in lateh machine then my thread can gets damaged but they are saying nowdays lots of people are doing the same thing.
Please advice what to do?
Regards
Anu
I have a problem with my 3 year old Honda unicorn 150CC bike. The bike starts but the moment I accelerate the engine stops. In the rainy season, this gets worst. I tried increasing idle rpm to 2000 but it doest work out. Some time during travel if I change the gears the moment I leave the clutch the engine stops and jump-starts. The service center does not seem to have an answer to the problem. I have tried changing the spark plug, cleaning the tank, cleaning the carburetor, changing the diaphragm in the carburetor. Engine de-carb. Nothing seams to work. The worst thing is the problem is not consistent. Does not happen all the time.
Please contact at sujeet.g@live.com