Although the principles below appear simple enough, without the necessary experience I'm afraid you may end up doing more harm than good.

I suggest first having the keyboard evaluated by a professional to determine if the bars are suitable candidates for learning to tune on. In the mean time you can use hardwoods from your local lumber yard to experiment with the cause and effect of bar tuning.

The following general principles apply when tuning all keyboard percussion instruments -- whether made of wood, steel, aluminum or synthetic composites:

1- Removing material from the center of the bar lowers the fundamental pitch.
2- Removing material from either of the two ends raises the fundamental pitch (and the higher overtones to varying degrees).
3- Removing material from particular locations alters the remaining overtones/vibrational waves.

For a detailed examination on the physics of keyboard percussion I recommend Thomas Rossing's book "The Physics of Musical Instruments". He also has an article written for Percussive Notes-Research Edition (Fall 1982) which is available online here under Articles and Archives.

Something to consider: Is it possible that during this instrument's history a previous owner, whether as a cost-saving move or out of desperation, decided one day to re-tune the bars himself?...and now you're dealing with the result.

John Salazar
Salazar Fine Tuning

Answer:
Joshua,
There is no simple answer to this question. It depends on how the bars were tuned in the first place, if wood was originally removed in areas that probably should not have been removed, if the playing surface is torn up, has it already been re-tuned and what did that person do. If you remove too much wood from outside the node, it will actually shift the node which will have the effect of dampening the bar, and many more issues. So, my recommendation is if it is a quality instrument, send it to a pro to re-tune.

Ron Samuels
Marimba One