The weather was good and it was abnormally warm today (around 15°C!), so I turned on the compressor and sprayed some colour. As I should have already written, my airbrush is corroded and:
1) the needle is hooked
2) the nozzle is slightly deformed
I usually spray acrylics, and noticed that even at 3-3.5 bars the colour, milk-like consistant, maybe a it thicker, does not flow well at all and it is impossible to spray big surfaces. Moreover, if I spray water or alcohol the covered area is wider.
I'm aware it is an idiot question but does this depend on the damaged components of the airbrush?

A corroded airbrush with a hooked needle and a deformed nozzle is not going to spray where you want, how much you want, or in the fashion you want. It doesn't matter what pressure or paint ratio you use. You really need to repair or replace the airbrush. Anything else is just an exercise in frustration.

Ditto, what Matthew said.

Very clear, thank you.
So my XMas present will be a new airbrush.

See how easy that was!