carb, air cut off valve

[yorkshire]

Hi All, I know there is a lot about carbs on this site, what does the air cut off valve do? I have had a look on the internet regards this but there is not a lot there. I have had a look at how constant velocity carbs work on the internet and I understand this but I have only seen simple CV carbs shown, with no air cut off valve.
The air cut off valve has a strong spring, there must be a strong force to open or close this valve with such a small diaphram
It would be great if I could understand this, Ian

[squirrelman]

higher vacuum during downshifts pulls a valve open,letting in richer air/fuel mixture to prevent lean backfires...... or something like that.

they almost never go bad or need replacement.

[yorkshire]

higher vacuum during downshifts pulls a valve open,letting in richer air/fuel mixture to prevent lean backfires...... or something like that.

they almost never go bad or need replacement.

Thanks for that squirrelman, that's good to know, I have a Honda ntv manual which is the same engine and it does not give a season for the air cut off valve.