Thanks to both of you, kirkkw & mjydrafter.
Even if I'm not sure that the problem is a clogged jet, cleaning the carburetor won't harm anybody, if done correctly.
Just I won't do it myself, as I am afraid that I might damage something. But for now the bike is at my country side, I assume that it's not very harmful if I'd still use it a little bit?
Also, just to be sure, you can adjust the fuel dosage with the jets, right? Maybe the main issue, the ''Reduced effect'' issue, was caused by carburetor, as I had to pull the choke fully out for the bike to be functional at idle.