ok, well ive had this little guinness whistle (my first one) laying around for awhile, and i havent played it cause it sounded (emphasis on ED) like a bird witha broken beek. so after taking a look at the stuff on the chiff and fipple webpage (go figure theres actualy content on the thing) i found the page for tweaking generations with wax in that space underneath the windway, and was looking at the mouth piece on my guinness (same thing as waltons little black whistle) which i was aobut to make a keychain out of, and saw it had the same gap, so i quickly got out the candle filled it in, it sound 110% better, not nearly as airy, although it still retains enough chiff to sound irishy, and there is alot more backpressure which i like… so i know its not much. but i thought it was alot of fun and i am fighting back the urge to do it to all my other plastic headed whistles.
If it’s not broke don’t fix it. Many whistles are great right out of the box. Tweaking can kill a perfectly good whistle. On the other hand if it won’t play, have at it.
Ron
You’ve only just begun. Be careful of the dark side of tweaking though, I became obsessed with tweaking whistles and didn’t really play for a few months. 
Come to the dark side…
I’ve actually got to order more whistles because I’m beginning to appriciat untweaked whistles now. Something about that raw tubular sound from a decent gen…
ya. i was thinking of going and buying a few of the waltons like i already have so i can tweak them and still have good ol’ untweaked ones.