I just lowered the bridge on my acoustic and my intonation is ever-so-slightly off, and it's driving me crazy. Does anyone know how to fix an acoustic's intonation?
it was my impression that you can't. the guitar has to be built correctly otherwise your fecked.
Quote by euphorictacos
are the strings old?

try some different strings or even perhaps change gauges.

Before taking any further steps rule out the possibility that old strings ruin the tone and then try different string gauches.

Intonation on a guitar is determined by the length of the strings relative to the 12th fret, string heigth above the frets and string gauche. Changing the action inevetibly changes the intonation too. Adjustment of the string length on an acoustic is not as straightforward as on an electric.

Always first check and maybe correct the neck curvature, for this affects string heigth also.
If this is done, see what intonation problems are left. Chances are that the octaves are a bit flat, for that is generally the result of lowering the action. Restoring the old situation might be a good idea. Or else, wear out the new strings (play a bit out of tune for a few weeks. That can't be helped if you don't want to bin your new strings right away) and next time try a bigger gauche. Don't forget to re-adjust the truss rod once again.

Pitty is that if you lowered the action to go for lighter playing, thicker strings will make it stiffer again. That's why I would advice anyone who wants to lower the action on an acoustic to check the intonation first. If this is spot on, either leave it alone or bring it to a luithier to have it set up to suit your personal preference.
Quote by gnomieowns
it was my impression that you can't. the guitar has to be built correctly otherwise your fecked.

No, it can be adjusted, but it's very tedius work and involves some expertise and knowlege of the mechanics of an acoustic. It's called "compensating the saddle" and requires one to file angles onto the crown of the bridge saddle to change where the string contacts the top of the saddle piece. Since the saddle needs to be removed to do the job, but it needs to be in place and the guitar tuned to pitch to check, well you can see how it would become very time consuming to do this job.
The suggestion above to start off with a new set of strings is spot on. This alone could remedy the problem in itself.