CNS Carb Tuning

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Maskot

Maskot

New Member
Apr 22, 2011
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Wellington, FL
I have a CNS carburetor and have no idea how to tune it. The slide needle has no c clip adjustment. The problem I've been having is when I have the fuel valve fully open, the max speed I can get is about 20. When I close it and let it run dry, then it goes over 30 and when it hits 30, it instantly loses all power (runs out of gas). I checked the spark plug, and I know it's running way way way too rich. How can I adjust this on a CNS carb with no c clip adjustability?
 
dodge dude94

dodge dude94

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Jun 8, 2012
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You have to take a dremel and very carefully but slots in the needle. That's how. No joke. The new gen CNS3 carbs are non-adjustable. I've got an older V3 so I've got all the screws and needle stuff. On the new ones you can drill out the little plug next to the idle screw and find the idle air mixture screw, that will help a bit. Especially off idle acceleration.
 
Maskot

Maskot

New Member
Apr 22, 2011
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Wellington, FL
dodge dude94 said:
You have to take a dremel and very carefully but slots in the needle. That's how. No joke. The new gen CNS3 carbs are non-adjustable. I've got an older V3 so I've got all the screws and needle stuff. On the new ones you can drill out the little plug next to the idle screw and find the idle air mixture screw, that will help a bit. Especially off idle acceleration.
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The c clip is at the very end of the slide needle, so I can't make it leaner by cutting new notches.
 
Maskot

Maskot

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Apr 22, 2011
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Wellington, FL
maniac57 said:
Try reading this:http://motorbicycling.com/showthread.php?t=38163
Then, if it does not help you, type "cns carb tuning" in the search box at the top of the page.
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Fire up and warm the motor, then while riding turn off the fuel valve, if the motor starts to run better for a short while before it dies (runs out of fuel), then you would probably benefit from a float adjustment.
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Looks like a float adjustment and air intake adjustment would be a good way to fix the problem, I'll look into it tomorrow and see if it can be fine tuned.
 
xseler

xseler

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Apr 14, 2013
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I've hit 33mph with my CNS V3 carb with a 44 tooth sprocket and a SBP Expansion Chamber. I've also adjusted the screw under the brass plug. The 'happy' speed is around 26 -27mph......just right for me!

Good luck in your tuning!
 
F

fonzie

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Nov 13, 2011
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I've fooled with many of these CNS carbs and they are all about air/fuel ratios.....while the c-clip will help you fine tune the carb, sounds like you are off pretty far. Float levels are another thing to check but again, unless you are spewing gas out of the overflow tube, probably not your biggest problem.Try taking the air cleaner off and run the motor. Chances are it will run much better. Now you can't or shouldn't be running around without an air cleaner so if that helps, get yourself a "known" number 70 jet(one marked with a 70). That should fix the problem. These jets from the factory are usually not marked and I've found them as big as a #80 jet. Way too big! I guess they are just drilling out any jet they have lying around and it can really cause tuning problems. Hope that helps. Oh, by the way, you should pop off the brass plug to access the fuel mixture screw. Start with the screw set at one full turn open.(screw it in the whole way till it stops and then turn open counterclockwise 1 turn)
 
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Maskot

Maskot

New Member
Apr 22, 2011
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Wellington, FL
fonzie said:
I've fooled with many of these CNS carbs and they are all about air/fuel ratios.....while the c-clip will help you fine tune the carb, sounds like you are off pretty far. Float levels are another thing to check but again, unless you are spewing gas out of the overflow tube, probably not your biggest problem.Try taking the air cleaner off and run the motor. Chances are it will run much better. Now you can't or shouldn't be running around without an air cleaner so if that helps, get yourself a "known" number 70 jet(one marked with a 70). That should fix the problem. These jets from the factory are usually not marked and I've found them as big as a #80 jet. Way too big! I guess they are just drilling out any jet they have lying around and it can really cause tuning problems. Hope that helps. Oh, by the way, you should pop off the brass plug to access the fuel mixture screw. Start with the screw set at one full turn open.(screw it in the whole way till it stops and then turn open counterclockwise 1 turn)
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I am spewing out gas when I open the valve.
 
Maskot

Maskot

New Member
Apr 22, 2011
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Wellington, FL
Pablo said:
You can't. The float just meters the fuel in the bowl. Has nothing to do with fuel/air mixture.
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It does, I figured out how to adjust it and so far it's added performance, some fine tuning and it'll be even better, I need it to run leaner, but there is nothing on the forum that I can find.
 
dodge dude94

dodge dude94

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Jun 8, 2012
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I found with mine that it was running lean in the bowl, so by basically increasing fuel pressure, by having more fuel in the bowl, it fattened up a bit.
 
Maskot

Maskot

New Member
Apr 22, 2011
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Wellington, FL
Pablo said:
Please explain what you did. Sounds interesting. Thanks.
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By adjusting the float, which limits the fuel or enables more fuel to go in the bowl, you can effectively adjust the amount of fuel that goes in to the engine by having the float close the fuel line sooner or later, to lean it up I made it close sooner so less fuel is in the bowl and less fuel will go into the engine.
 
Pablo

Pablo

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Dec 28, 2007
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Interesting. But still the fuel is sucked up through the jet, which really meters the fuel. I guess the volume of fuel in the bowl could slightly have an effect on flow volume. Still better to have a proper bowl amount and meter by the jet.
 
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