If I leave a Pokemon with the item Power Belt in the day care will it Ev Train ?I meanIn the day care they raise the Pokemon's level as much as you keep it there.If the Pokemon holds an EV train item, will it Ev train by leveling up ?
No. It is best to EV train before and then put the Pokemon in the Daycare after. Source: Experience
Basically there is a vague reason:
The Daycare will never know which stat you want to EV train in, so they will EV train randomly at no pace. That is pretty much it. You can have MAX of 510 EV's in one Pokemon and MAX 252 in one Stat. If one stat is already maxed out, it will not need any more EV training.
EV Training Guide then How to EV train. and Youtube Video on EV training. also there is Daycare Info.