How do I adjust the truss rod on a jazz neck that had the screw at the bottom?

Sorry to tell you but you will have to remove the neck.
Some just loosen the screws, but I would suggest taking it all the way off.
If you find you have to re-set the neck often and can't hang with the on-off thing seasonally, then get an aftermarket pickguard
that is notched for heel truss access.

 

Loosen the strings and unscrew the neck plate just enough to be able to bring the neck up so you can make your adjustment. Then reseat the neck and rescrew while maintaining some pressure at the neck pocket. It's inconvenient but not as scary as it sounds.

Someone posted a brilliant idea of trying a paint can opener, see if you can sneak one without having to partially or fully decoupling the neck.

Whatever you try make sure the tool is the right thickness and width for the truss rod and try not to chew it up or anything.

Once you get set up you won't have to deal with it that often.

Good luck and remember there's no shame in taking it in if you are really intimidated.

 

Purchase a Fender truss rod tool. They are not expensive and do the job without removing the neck or loosening neckplate screws. Very simple solution. I have owned my tool for over 40 years. Works every time.

 

Fender truss rod tool is hex shaped,
you'll need a flat blade screwdriver for that trussrod. Take the pickguard off, that might give you enough room to make an adjustment. Take a long flat blade screwdriver. Put the bass across your lap and put your arm across the body. Push down on the headstock with your other hand(don't make the trussrod do the work) and give the rod a 1/4 turn clockwise(to take out relief and counter clockwise to add relief) check adjustment, repeat as necessary.

 

Zero chance this method will work in this case.

Look at the photo of the bass truss rod in question. There is no space for inserting a truss rod tool and turning it without either loosening the neck, or else removing the neck or the pickguard so as to get access.

^^^^^

This

 

Using a flathead screwdriver to turn the truss.

It can help to stablize the bass between your legs as I am doing in the 3rd photo,
and then bend the neck a wee bit before tightening the rod. Just makes the rod have to do a little less work.

I used to have my '62 Precision set up at Eric's guitars, a very fine shop in Los Angeles, and the assistant tech told me on one occasion he
set the truss, screwed the neck back on and then still had too much bow and had to do it all over again. So even for an experienced tech this job can involve a bit of trial and error.

 

Here's one more tip for you, and this came from Eric himself. Anyone that knows that dude knows he's a sweet and mellow surfer type, easing going... So when he voiced a VERY strong opinion to my question, I took note. Basically, I asked him if one should turn the truss a 1/4....see how that goes... I've even seen guys on here suggest doing it a bit at a time over a couple of days. Any truth to this? "Nonsense!" said Eric, and "Total fallacy. Set the neck where you want it and don't be timid about over-turning. Do it with confidence and be done." Well, good advice for a guitar shop, anyway, where you got to get 'em on and off the bench promptly. But that's part of why it is cool to set up your own bass, you'll really get in to optimizing your setup so it is just right for you. Hell, Jaco used to stop the show and re-intonate on stage

 

Once last clarification -- Referencing the orientation shown in this photo:
If I wanted to make this board flatter, I would

a) hold the neck firmly between my legs as shown
b) put my right hand against the fretboard and push it toward my left knee
c) turn the truss clockwise with the screwdriver using my left hand while maintaining hand and leg pressure.

If your neck has too much backbow and you want it LESS flat, I would turn the neck 180 between my knees, push in the same direction as before but turn COUNTER clockwise

 

I adjusted mine just by removing the pickguard, you just have to find the right screwdriver that grabs the screw real good and doesn’t touch any other areas of the bass..

 

with a screwdriver

 

It’s nonsense removing the neck. No need. Loosen the screws on the neck plate and put a little pressure on the neck. It will angle up far enough for you to make your adjustment. I have a Fender 60s J with the trussrod access at the heel also. It was no big deal.

For future reference however, I would purchase a Telecaster trussrod tool from StewMac. Great tool for this type of application. It should fit in the little gap. If not, do what I did and take it to a reputable person who can “notch” that little gap just a hair more. Then the tool will fit in there very easily and your future trussrod adjustments will be a breeze.

 

When doing a similar adjustment on my guitar I use a capo to hold the strings so I dont need to remove them completely. Very convenient. IME these adjustments are very rarely needed on good maple necks anyway.

 

Climate and environmental changes (relative humidity) can yield the need for several adjustments each year. I have experienced this first hand. If the OP lives in a state that goes through “the seasons”, he will be adjusting the relief at least a few times a year. At least I need to.

 

I always considered the 1/4 turn rule as a device to help newbies who may be fearful, get more at ease and develop an understanding of what to expect. It would also prevent the occasional idiot from just torquing away until the 8th turn pops a weld.
After following the 1/4 turn rule for my first few attempts at truss rod adjusting, I developed a sense. Now I look at a neck and my first adjustment can be anywhere from 1/4 turn to 1 1/2 turns, depending on what I see and how it reacts. So yeah, your experienced tech, or anyone who’s done it more than twice, comes to understand.

 

Most old Fenders (and some re-issue ones) all have this problem. Fender fixed it later.