Auxiliary Air Valve wanted please

Hi, I'm pretty sure my auxiliary air valve is playing up.. typically as I just put the little car up for sale! Does anyone have one as inexpensive as possible pretty please?

You can sometimes just clean them out with carb cleaner, remove the valve and plenty of soaking some times un sticks them?

I have one if needed

Hi,
Thanks both. I removed it and it was open. Cleaned it anyway. Problem rights itself after 5 minutes so think it's working but I must have an air leak some where….