New CNS Carby: What is the 'Vent tube'?

RLorange

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Jun 21, 2008
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Australia
Hello everyone,

I have purchased the new CNS style carby which is working great but I am still fine tuning.

Very little info on this carby although I have worked it out from my knowledge of carbys in general... except one thing.

The CNS has a nipple at the bottom of the fuel bowl at comes with a crappy hose attached. Inside the bowl this nipple extends up well above the fuel level so I am assuming that it is some sort of overflow. Where is it supposed to overflow to? all over my bike and engine? Grubee Inc says that it is a a 'Vent tube' and should be left open. I have seen a photo a while ago that shows this tube joining back up to another nipple which leads to inside the venturi. Grubee says this is not used.

Seems strange to have an this tube thing at all does anyone think it would be safe to block it up?

Other than that the CNS is a worthy upgrade. Much improved acceleration and stronger top end too although fuel consumption seems higher, maybe this will drop once it is fine tuned to perfection.
 

jasonh

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Jun 23, 2008
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Longmont, CO
if it's poking up over the fuel level, I'd guess it is for fuel vapor. When you say 'venturi' are you referring to the air intake?
 

RLorange

New Member
Jun 21, 2008
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Australia
Yeah there is a nipple that extends into the intake, Dellorto call it the venturi sorry. No Idea what this is for it seemed logical to join up the vapour vent to this before I knew what it really was for.

Ok so I'll leave the vent open and ignore the nipple into the intake.

Cheers.
 

stv1jzgte

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Feb 11, 2009
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australia
Yeah there is a nipple that extends into the intake, Dellorto call it the venturi sorry. No Idea what this is for it seemed logical to join up the vapour vent to this before I knew what it really was for.

Ok so I'll leave the vent open and ignore the nipple into the intake.

Cheers.
Sorry i know this is an old thread but im haveing a similar problem.
Should the nipple to the venturi be blocked and then run the hose from the float bowl to atmosphere?
My carby kit came with a hose from the lower nipple to the npple in the venturi also. It runs fine but will not restart once it is hot.
Thanks steve
 

Tim_B_172

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Aug 26, 2009
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Saint Joseph, MO
The tube should hang down so that any overflow will go onto the ground rather than on you're engine. The nipple in the venturi doesn't need to be blocked, I've run mine with and without having it blocked and I don't really see a difference. I think the purpose of that nipple is to inject something into the airflow, like nitris oxide. Not a good idea if you ask me, but that's the only purpose I can see for it.

Hey I just got an idea. With a fuel pump, and some kind of pressure regulation for it, you could use that nipple to run this carb as a throttle body inject set-up. Wouldn't that be cool?
 

Tim_B_172

New Member
Aug 26, 2009
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Saint Joseph, MO
Also:

Just a general maintenance tip for that carb. That filter will soak up oil, but you can't see it, so as I learned recently it can get so saturated that it will choke the engine. So if you start to experience any lack of power or poor performance, wash the filter with dish soap and hot water. Let it dry completely before you use it again.
 

john_the_great

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Oct 24, 2009
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California
The tube should hang down so that any overflow will go onto the ground rather than on you're engine. The nipple in the venturi doesn't need to be blocked, I've run mine with and without having it blocked and I don't really see a difference. I think the purpose of that nipple is to inject something into the airflow, like nitris oxide. Not a good idea if you ask me, but that's the only purpose I can see for it.

Hey I just got an idea. With a fuel pump, and some kind of pressure regulation for it, you could use that nipple to run this carb as a throttle body inject set-up. Wouldn't that be cool?
That one nipple is for oil injection, the yamaha cy-80 is the bike that used this carb and it had oil injection.
 

AussieSteve

New Member
Nov 26, 2009
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Nowra, NSW, Australia
That tube is actually, as far as I know, a 'vent' tube as described.
It equalises pressure in th float bowl. (God knows why, but I've struck a number of carbs over the years with a similar setup.
 

stv1jzgte

New Member
Feb 11, 2009
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australia
Lets not get our tubes crossed, the 2 straight brass tubes out of the bowl are vent or equalizer tubes
the lower nippled tube is an overflow and the barbed ventruri tube, oil injection as John_the_great said sounds about right.
 

john_the_great

New Member
Oct 24, 2009
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California
Lets not get our tubes crossed, the 2 straight brass tubes out of the bowl are vent or equalizer tubes
the lower nippled tube is an overflow and the barbed ventruri tube, oil injection as John_the_great said sounds about right.
Thanks fer backin' me up stv! Assuiesteve, your right about the 2 straight brass tubes but the barbed one, like stv said, is for oil injection.