For instance, let's say a Pokémon had Pokerus, was given a Power Weight, and KO'ed a wild Azurill (1HP). How many HP EVs would it get?
Power items increase Evs by +4.
And your Pokemon will gain (1+4)x2 Evs that is equal to 10. So you'll get 10 Evs for every azurill you KO.
Hope I helped :)
Source: knowledge And experience