Can anyone having success tweaking Clark tinwhistles advise me if I should tighten (slightly close the windway) and straighten the lip? I can image one could close the windway, but that looped lip is something else!
Second question. Are there any of you who may have heard of folks making a vent hole for octave jumps in a penny whistle? Andy
This is what I do to my Clarke originals. It keeps the same sound and lessens the breath requirement. Just be careful that you don’t let the sides bow out and break the seal when you push down on the arch.
It seems to me that when you push the windway top down like that the second octave can get really breathy. But it will give you an easier playing whistle I guess.
That’s what has happened all the times I’ve tried it..then again, that was like nearly a decade ago. I may have better luck now that I’m not such a n00b 
I pushed the top of my Clarke original down, and it became breathy in the second octave. Then I pried it back up, an lost the focus on the first octave. After a couple more adjustments up and down, making the top edge of the window a bit wavy, it came out right!
You haven’t lost unless the windway actually cracks! Keep trying!
Do you mean like a thumb hole on a recorder?
The end that the air goes in does not need to be closed. What makes the tweak work is flatning the air exit to the blade. Then the blade will need to be moved up or down to catch and split the air stream just right.
The other thing that will improve the performance of the whistle is to remove the wood block and make the top of it glass smooth. This can be done with very fine emery paper, 600 or higher. Then wet it, and let it dry then sand lightly again. Wetting the wood makes the grain rise. Then sanding brings it smooth. Do this about three times and the grain will stay smooth. A very small bevel can help at the end of the block.
Fearfarion check that spelling.
Andy, did you try making the windway surface of the wood block smooth?
Put the very fine emery paper on a piece of flat glass to keep it smooth then rub the block on it softly. Wet the block and let it dry. This will raise the grain of the wood. Then after three times it should be real smooth.
To make it more smooth after useing the emery paper. Take piece of brown paper bag and put it on the flat glass and rub the block on the paper till the wood shines. You can find a piece of flat glass hanging on the wall.
Mine was good after having just pressed down on the top of the windway exit with my thumb. Then I slightly bit down on the same spot and it became even better. Finally, I bit down on it again, but this time its tone bit the dust. Now sounds like a harmonica/accordion throughout its second register. Needless to say: Don’t bite it. I’m not sure how far they can be pressed down before the same thing happens.