Thank you, my printer is Selpic Star A, as mr. FooBar one's.
When I tried to tighten the pulley I noticed that some of the bed screws were too loose so I tightened them a bit.
I decided to do so to fix the bad result (1st vase image) and because mr.foobar suggested to do so in a post.
I really didn't know how much I should have tightened the pulley (the regulation appear under the bed) but it seems to be a bit looser than the X axis pulley, I'm going to check all and update the firmware to Marlin 2 that allow to change accel params without reboot, the current values I used are the default by vendor fw.