Modern Vespa

I take a new Spaco carburator SI 20 20d autolube for my Sprint Veloce.
Since i have problem to optimize the carburation (too rich at idle, mixture screw at -0.5 out) and lean to medium and high rev, i would like to know what is the hole near the main jet. As you see on the picture, the correspondig old Dell'Orto carburator has not the hole.
Is it possible to close this hole?
Thank you

Try spraying some carb cleaner in the hole, and see where it dribbles out of some other orifice. Might need to hold the carb on its side to see where it flows.

I have tried to inject alcool, the hole seems to be blind, alool did't dribble out, so i ask me why there is this hole ? Have you an explanation ?
thank you

Did the carb come with an air filter.

I'd like to see the underside of it.

Maybe theres a pin that assures fitting or I also see a channel from the hole to the venturi.........anyway, post a pic of the filter.

The carb did not come with an air filter and is a new Dell'Orto/Spaco carburetor SI 20 20 D cod. 590 for Piaggio Vespa PX 150 autolube with the seguent configuration:

main jet: 100
slow running jet: 48-160
diffuser: BE3
main air bleed: 160
throttle valve: 6823.1
starter jet: 6o

The air filter i use is the same of PX 150 and it cover the hole without pin.
However the channel you see from the hole to the venturi is free with the air filter installed while the hole is closed.

i have just bought and fitted a 20/20 d non autolube and it has the same hole no idea what its for tho
main jet 102
mixer be3
air bleed 160
idle jet 48/160
slide 1#
dellorto have a strange way of sizing the idle jet you divide the large number by the smaller number god knows why