Eh, Sanwa JLFs feel crazy loose with the original springs too. It's just a trait of the Japanese style sticks.
I've read that Japanese players don't hit restrictor like I do, they only move the stick as much as is necessary to engage that direction.
See how long and wide the spring is, then see if a 3-6lb Sanwa JLF spring would fit.
IIRC, I have 3 or 4lb springs in my JLFs.