I have a chieftain low D whistle and the C# is pretty flat. Does anyone have a fingering suggestion? Please post.
If it’s the C# (not C=), I don’t see what fingering could help. Try blowing a bit harder.
Or contact Phil Hardy here on the board, see if he can revoice it.
If it is really the c# (ooo ooo), then Zoob has got it right: blow it into tune. If you in fact me the c-nat, then try using this fingering: oxx ooo, rather than the traditional oxx xox. If even oxx ooo is flat, you can go to oxo ooo which is noticeably sharp on most whistles but I think was used by some of the older players in Ireland.