Bad neck?
 

Hey hey. I have a lot of posts in the Warmoth-building thread, but this post has more to do with the bass I already have: a lefty 2004 MIM Fender Jazz Bass.

I'm wondering if the neck is bad, because even when I put lighter gauge strings on (Ernie Ball Super Slinky), there's a pretty severe bow in the neck. Between the top of the body and the nut, it bends much like a bow-and-arrow. I have tightened the truss rod very far, and am afraid to snap it if I go any farther. Even as tight as it is, from the side profile, it appears mostly flat until the fifth fret, and starts sinking down at the sixth.

Other pertinent information:

I've switched out my pickups twice. First, I changed out the mexican standard-issue Jazz pickups for the American version. With the American ones, there is a warble in the higher frets, especially on the D and G strings (unless I drill the suckers down UNDER the pickguard). Next, I tried Seymour Duncan Quarter-Pounders, which don't seem to have the warble very much, if at all. Still, I've heard that such sonic malfunctions can be caused by a bad neck.

Anyway. Bottom line: should I replace my neck? I've been researching Warmoth and really like their 2 extra stiffening rods idea. Does the Fender neck only have a truss rod?

Sounds like you need a spacer washer under the rod nut so you can adjust it further.

I have a mim neck about the same era as yours. Same thing. I've read a few threads a while ago in which others had the same problems. I guess that's how they made them back then :/
I've got washers, even cracked the neck twice at the first fret from over tightening it and there is still a good amount of relief.

Ouch sorry to hear that!

Have you tried taking the strings off, putting back bow into the neck (e.g. resting the body against the wall, holding the neck around the 3rd fret, and pushing your knee on the neck plate), and tightening the nut? I have a neck that could only be tightened that way. Of course, it's risky business; but if you go slow without any bouncing, you should have good results and no cracks/creaking.

I had the same problem with an '80s Squier. I gave up and got a Warmoth neck.

I would suggest back clamping the neck into a a backbow, adding the washer, & readjusting. If that fails to produce results, heat treating the neck may be your last line of defense.