hi. i have somewhat a good understanding about the carburetor passages. but it has a port which is kind of hard to understand.
we all know that we have to have some vacuum passage to activate the diaphragm. just like the below picture (picture was taken from a Youtube video).



so in our carbs it's clear that the hole "A" is related to the choke valve. but there's another hole which is "B". when we check it from the other side it opens to the venturi just behind the butterfly valve. is that the port which helps to lift the diaphragm using the vacuum created by the engine ?


it's bit confusing. reason is when we put the rubber seal and vacuum chamber cover over it, they restrict the connection between the top part of the diaphragm and that hole completely. you can see it by the "C" area.


so how that vacuum affects the diaphragm and makes it lift. i read this article by IDODirt. it's a very useful post. but it doesn't explain how the diaphragm is lifted. can someone explain it please. thank you very much.

 

well i was wrong! just discovered that the port (B) is the air passage for the choke circuit. IDoDirt had mentioned it in his article but i think i missunderstood it.

anyway since it doesn't explain how the vacuum slide lifts i read some other articles. according to them these two small holes are what make the vacuum slide lift


but again when we put the jet needled holder there the both holes get covered.


just wonder how that two holes make their way up to the vacuum chamber cover so the vacuum slide can lift using engine's vacuum.

the below picture shows how the engine vacuum affects the vacuum slide in a CV carb exactly.


if someone can explain how it happens in our carb, it will be very useful. thank you.

 

It happens exactly like you see in this image above. The two tiny holes at the bottom do get covered up visually by the holder, but not sealed. Air (or vacuum) is free to flow (albeit metered by their small size) in and out of those holes and that is what exposes the upper portion of the slide to vacuum. That lower pressure allows atmospheric pressure to overcome the slide spring and lift the slide. Then as the slide lifts it enlarges the throat of the carburetor reducing the vacuum to the point where the vacuum, spring pressure, and atmospheric pressure balance out.

The slide doesn't spend much time in a raised position or at least not very high up. It's usually only under wide open throttle operations or at least heavy throttle that you see them raise up really far.

Good questions and it's nice to see someone actually trying to really understand it.

 

thank you very much learned a lot from your article about carb ports and only thing i couldn't understand was the choke circuit and this slider thing. the reason is i misunderstood the choke air passage. now i fully understand both of them. thx again and much appreciated your article as always

 

Maybe you could paste a link to the video that you got the image from? I'd be interested in seeing it. The graphics are good.

 

sure