A problem I have been experiencing with my M14 (KART JAE) is that when firing on semi, it will lock or continue to shoot on auto. To fix this, I would switch back to auto, fire a few shots, then switch back to semi. So I recently opened up my gun to see what the problem was, and saw this:

Broken selector bracket:


Wires soldered instead of using connectors:



Now for the 2nd pic, the wire actually came off, so I had to resolder it. My question though is should I buy these replacement parts:
M14 selector bracket
selector switch
wiring harness
OR should I just buy a whole new gearbox
AND my last question: will this solve my ROF mode problem?

you can use better wire if you want but all you really need is the selector bracket.

after charging my batteries, I realized the gun still works ( just semi), so I'm just going to replace the bracket for now

Your fire mode problem lies in the selector plate and yaw assembly. The trigger switch would just not work at all. Possibly buy a new switch assembly, but I would just get the new selector plate and correct the yaw timing. If the yaw is dysfunctional...then you may need a whole new mechbox shell or a competent tech with a dremel to make adjustments.

Good luck.

EDIT: I am going to add. if you have solder work in close proximity like that in your second pic. It is best to wrap it.

Thanks for the help guys!
@lonewolf, you know a lot about airsoft tech and I wanted to get into it also. How did you learn how to work on gearboxes and other internal parts?