I had the toothed pulleys ready (16T), I changed the flat ones, nothing has changed... Ok, the 16T aren't perfect with the motors 20T pulleys (the belts parallelism is not perfect like before), but I don't think that make a different for the VFA.
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Now I'm trying to make a rubber base for the motors to try to "filtrate" the vibrations, I'll keep You informed but I'm not so confident...