wait, first, did you ever jumpstart the psu to see if it works? also have you tried it without the 24pin adapter? just a little info on it:
20pins: was industry standard
24pins: new industry standard
extra 4 pins: power for high power consumption expansion cards
soo....
try not using the adapter and turn it on with your video card out.
also, jumping is not dangerous, all you are doing is making continuity between two points that triggers a switch inside the psu to turn itself on. your just messing with grounds
but still, check the psu in your old build, and also try to jump it
EDIT: and last thing, if the psu works, but not with the new motherboard, check your cmos jumper.