My D chanter plays well but is about a quarter step flat in pitch. What
adjustment to the reed should I make to correct this?
To raise the pitch, shorten the length of the chanter. This usually means moving the staple into the throat further (usually by just a tiny fraction of an inch). You may have to remove a couple of windings of thread from the bottom of the staple. If you can’t get the staple to go in any further, check to see what kind of rushing you’ve got going on inside the chanter to see if something is bringing overall pitch down.
djm
If the reed plays well in tune with the chanter but you like to have the chanter in tune to play with other pipers or fixed pitched instruments.
You said the reed was quarter step flat in pitch. What I would do is move the reed further in the chanter throat this will sharpen the reed.
See Charles Roberts web site for tuning your chanter http://homepage.tinet.ie/~robertscharles/tunechanter.html
and also read The Uilleann Pipe Reedmaker’s Guide by
Dave Hegarty he has sections on reed problems and tuning.
All the best