That would be a valve to adjust idle, as to obtain a proper air/fuel ratio. With this set-up you have absolutely NO fuel from a pump or any other source. This will run what you want. You only have to adjust the size of the ports according to the needed input-output.
The more you close this valve the more fuel you will take in as vapor (or more vapor you will make), the more you open the valve the less vapor you will make! Very simple and very effective for light load apps.