So, I went by the garage the other day and i’m starting to think this will not be an easy fix.
I checked the manifold before turning the engine on - it seems deflated, same as it is when the car idles. so it only “inflates” when i accelerate.
Also, I opened the airbox and i got the same result as if the box would be closed (apart form the fact that it was even louder). so this didn’t solve the problem.
In addition, i noticed the engine is quite hot even after a short drive and as far as i know this can indicate too much air in the mixture. The temp indicator in the dash was still within limits thought.
i did look down the main hose to see the butterfly and all seems fine, nothing stuck in there.
At this point, after what i saw and after you guys told me, i think i will have to take the car to a mechanic. it might be that the plastic manifold is indeed faulty and that’s not something i can change by myself.
so i guess that’s that… thanks for all your help!