hey.. my friends strat has a very low output lately ive noticed.. it sounds like a volume knob rolled to 3 (so it totally sucks for metal and anything in general). I thought it could be a pickup thing, but i think its a problem with his jack.. it always comes loose, but he tightens it.. is there anything i can do, like unscrew it and fix anything?

check the wiring to the output...the solder may be coming loose. And then if that fails adjust the pickup height and such.
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1st you do the easiest, that is to check the pup's height. Fret the last fret and try raising the pickups high enough, not touching the strings and see whether it has any changes. If not, then screw it back down again to its original position coz it may be your jack/volume pot. After checking the jack, check the electronic's compartment, connection/soldering.

If it is your jack, then do these simple steps.

Tightening the input Jack
1) Open up the input jack's plate, do not tighten it from the outside coz it will twist and snap the connection inside.
2) Insert the jack, see whether the connection is proper, then only you tighten it. Make sure that the shield and tip connects to the hot (red). Also, tighten it properly so that you dont have to do it again. Always good to use plumber's tape if its loose.
3) Test out which wires go where by tuning ur amp on if you're not sure. Resolder the connection.
4) Screw back the plate.
"Play with your ears" - Yngwie Malmsteen, Paul Gilbert
Thats what she said...
could be that one of the wires connected to your jack is starting to strip away since your tightening from the outside and not holding the inside section
low output compared to what? a humbucker equipped guitar? a strat has a lower output than a humbucker guitar if it's on stocks because the magnets aren't as strong.