I’m not sure if this would fall under your definition of “tweaking”, but I’ve tried making some quite extensive modifications to one that
I bought used, for next to no cost, on eBay. First, I used my Pratten reamer to completely reshape the bore. This made a big difference
right away. Then I made some minor modifications to tone holes, undercutting in the appropriate places to improve tuning balance. This
worked quite well, but only because the tone holes happened to be in the right locations to begin with. In fact, I only did this work because
I measured all the relevant parameters and determined that there was hope ahead of time. You can’t really know this until you have the flute in
hand, of course!
The embouchure cut was poor, and too large, but the flute played quite well after reaming and tone hole tweaks. Ideally, though, it would
have had an embouchure insert and a new embouchure cut to make it play really well, but I didn’t do that.
The other issue was that the keys were poorly made and fitted. I spent a long time tweaking the springs and replacing key pads, and eventually got
it working quite well, aside from one troublesome and unpredictable leak which was due to the key slot for the short F key having been cut
poorly. Specifically, it was tapered and slightly too large, such that the key wobbled, sometimes leaking and sometime not, all depending on
how you may or may not brush against it while playing. Uh! After some tricky shimming I got it working moderately well, but I suspect these
kinds of problems are common, and they are horrible to deal with. Fitting keys well requires a high degree of skill and practice, and is one
of the reasons why keys cost so much on a good flute.
Surprisingly, the wood was actually very nice on the flute I had. It was probably African Blackwood, but I’m not 100% sure. I had it for several years
and it was very stable. I don’t think this is a feature you could rely on though. I think I was just lucky with that one.
In the end I got a reasonable 5-key flute for my efforts, but it was a lot of work, required a flute maker’s workshop and tools, including appropriate
reamers, and was definitely not the kind of work the average player would want to undertake. The time I put in would have been better spent making
a flute from scratch, in the sense that I would have had much more control over the final outcome, but I did it mostly to satisfy my curiosity.
I did originally wonder if there might be an opportunity to apply predictable and repeatable tweaks in order to make some of these cheap, unplayable,
flutes into inexpensive keyed starter flutes for folks who can’t afford a new keyed flute. But I think the real issue is one of consistency, or lack thereof.
If you bought a whole load of these flutes, there is no guarantee that they would all be identical to each other, or even very close. I suspect it would be
a real crap shoot in terms of what you would have as a starting point. This would mean that the work required to get each particular flute into decent
playing condition would likely vary from one flute to the next, ranging in difficulty from being fairly straightforward to totally impossible, with a median
point somewhere around “troublesome”.
Overall, I’d say that it is not worth the hassle, unless you are doing it purely as a learning exercise and are happy to have a high chance of
ending up with an FLO that doesn’t play well.
The other side of this is to recognize that the devil is in the details, and making a good flute, especially a keyed one, is much more difficult than it
seems. The cost of the materials, such as the wood, head lining, metal for keys, is relatively insignificant compared to the labor cost. When you
try to tweak an inexpensive keyed flute you are really getting no more than the raw materials, and they will likely not be high quality (poorly seasoned
wood, poorly plated brass rings, etc) and may have been irreparably damaged by the original maker.