What makes Spears do double damage on Jump?

[Gi Nattak]

Anything in-battle related is handled in the C2 bank, so I'd look in the C2 disassembly first.
Searching for 'spear' brought up this:

Set $BD to 2 if A is between #$1D and #$24 (inclusive)
Increment damage if weapon is spear

C2/1512: C9 1D CMP #$1D
C2/1514: 90 08 BCC $151E (Exit)
C2/1516: C9 25 CMP #$25
C2/1518: B0 04 BCS $151E (Exit)
C2/151A: A9 02 LDA #$02
C2/151C: 85 BD STA $BD
C2/151E: 60 RTS

Then you can see from more instances of 'spear' searching that the STA $BD (the stored incremental damage) is placed in the Jump code:

C2/1817: 20 12 15 JSR $1512 (Set $BD to 2 if spear)
C2/181A: BD A9 3C LDA $3CA9,X (Weapon in left hand)
C2/181D: 20 12 15 JSR $1512 (Set $BD to 2 if spear)

So that's basically how it's done. How to go about changing it to another weapon though, I'm not exactly sure except some value in the 'Increment damage if weapon is spear' code must be set to spear. Maybe someone else can help clarify this more.

EDIT: What are you trying to do though exactly, change it to another weapon? Remove it?
If you want to just remove it, I'd imagine just NOP (EA byte) the two JSR there in the jump code would do that.

Oh and in case you are not sure where to find these disassemblys, you can find them on Slick Productions site here:
http://www.slickproductions.org/docs.php?id=FF6