I have a delron flute, no keys. It plays fine and is intune with itself octave for octave but the open C# is flat and is requiring a large adjustment to play it in tune. The c natural is fine. What can be done to fix this? I know that undercutting the first hole would bring it up but it will also effect the c natural. the octaves are fine so I don’t think and adjustment in the cork will do anything without screwing up the octaves but I may be wrong. I am new to playing the flute so I was wondering if I should give it time while my embouchure develops. Any thoughts?
Yes. You need to lip it up.
I suppose that’s what the OP means with “requiring a large adjustment to play it in tune”…?
Yes. All flutes require minute adjustments to each note to play the notes in tune and with the best timbre. Mainly lip configuration and air velocity affect intonation; it’s the combination of the two that create each note’s volume and intonation. The c# in Irish fluting tends to be a little flat. It’s OK–it’s part of the distinctive character of the Irish flute sound.
PS–don’t mess with the flute itself. If it’s from a good maker, you should play it for a long while to see if you can play it in tune, and then you still should not mess with it. 
Thanks for your fast reply. I will continue to work on my embouchure and “lip-up” the c# when it bothers me.
What he said. I have this problem with a couple of mine, you get used to it fairly quickly.
The C# is flat to give a decent oxx ooo C nat. Undercutting wouldn’t change much - you’d have to enlarge the hole altogether, which might well mean needing to use a different (flatter) C nat fingering (oxx xoo or oxx xox), though you probably wouldn’t lose the C nat altogether provided you didn’t overdo it. I have done this, very cautiously but successfully, to a couple of Seery Delrin flutes which have passed my way and had intolerably and unnecessarily (and, in playing rather than testing mode, unlippably!) flat open C#s. There is an inevitable design compromise involved due to the triple purpose of the L1 tone-hole, but some makers make a choice there with which I would not concur, and others makers manage to get a much better result.
It might be worth trying fingering the C# ooo xxx instead of all open - that is a fingering found in period charts and on some flutes it noticeably sharpens the C#.
You are quite right that adjustment of the stopper cork will have absolutely no effect on this problem.
All the above said, I do agree with the general opionion so far that, as a newbie, you do need to develop your embouchure more before even contemplating taking a file to the tone-hole! On the other hand, if it remains a persistent, distracting problem, you will need to tackle it and not be forever struggling with it in a way which puts you off or impedes development.
I had a similar experance with a Delron flute which I had purchased new (back in 2010, I think it was). I thought it was me for the first few days. Then I took it to my flute tutor, a professional musician with many, many years of flute playing under her belt. She tired out the flute and quickly determined that it was the flute. I contacted the maker by phone and told him of the problem and he said that I could return the flute for a full refund, no offer to make the flute play correctly, nor any explaniation as to why the C# was so flat. Well, I did just that and received an immediate full refund. I was quite disappointed as I really liked the flute, with the one exception of the flat C# which I couldn’t tolerate. 
I might make mention that I carefully had researched the flute and had read many posts on this board concerning how great that particular flute was. I still see posts which extol this particular maker and his instruments. Today, you couldn’t give me one of his flutes as a gift, let alone purchase one.
Thats when I contacted Casey Burns as I wanted a keyless D. I explained my problem to him and he assured me that his flutes played in tune. His advice was to go to a maker who knew what he or she was doing. I took his advice and since have a couple of really great flutes made by Casey. Casey does not make Delron flutes due the gasses released from the plastic during turning and the impact it has upon his health, but he sure makes great wood flutes. Are there other good makers out there? Well, the answer is of course, yes. My point is to communicate your needs/ concerns with the prospective maker of your flute. If you have particular issues, discuss them with the maker and see if he or she can make an instrument that will suit your needs. A good maker, in my opinion, will always take the time to hear and understand what your needs are, if they don’t move on quickly and don’t look back.
My advice is to seek to return the offending flute, get a refund and find a maker who can produce an in tune flute. After all, when you purchase something as personal and important as an instrument you should have every expectation that it is right and proper. Another important lesson I’ve learned is that you get what you pay for. Now, of course, if the problem resides with the player, that’s a different matter; that having been said, it’s not unreasonable to expect that a flute will be in tune. In my opinion, having to “lip in” a very flat C# is beyond the pale.
And you’re suggesting that this other maker (who you’ve as good as named) doesn’t?
I guess i am dense, but i can’t guess the maker of the Delrin flute..
Also, the Delrin flutes i have seen, heard and played were definetly made by people who knew what they were doing.
… and if they didn’t, they didn’t mention this on their website? 
Or did you know they didn’t but ordered a flute from them nevertheless?
My experience was learned the hard way! If you buy a decent flute from a respected maker it is most likely ALWAYS you that need to improve, not the flute. My first flute was a Gert Lejeune flute and I was a complete beginner. I could not get the tone or intonation at all, and put it away for many years. Then I started all over again and still blamed the flute when I couldn’t get the sound I wanted:-) Sold the flute, bought a M&E Ebonite, then a Tipple, then a Marcus Hernon, then a Sam Murray and finally a Martin Doyle. Surprisingly enough my tone and intonation improved during these different flutes but it had nothing to do with the intonation of the flutes. It was me improving!
So if I had stayed with the Lejeune, and focused on practising rather than looking for the holy grail I would have been a much better player today! I wasted a lot of time:-)
No need for guesswork. It’s Dave Copley, because psychodonald’s told us about his C# issue before. But not previously suggested that Dave doesn’t know what he’s doing!
Now, I’ve got two-and-a-half Copley flutes (two heads and three bodies) which I adore, but would agree that Dave’s C#s tend to the flat side. Which hasn’t worried me greatly when I’ve coincidentally got C nat thumb holes and instinctively vent them Boehm-style when it bothers me. But I’ve also consistently found Dave helpful beyond belief, would have no hesitation in discussing their tuning and possible resolutions with him if I did want something done and, from considerable experience of how far he’s already gone for me, expect him to treat this seriously. His flutes might not suit everyone, but he’s both a master flute maker and a thoroughly decent person, and some of psychodonald’s comments are quite simply not fair in my book.
Interesting. I had a Copley early in my fluting journey, and I found the C# very flat.
I now have one of his Delrin flutes and couldn’t be more pleased with it (it’s my second, the first had the modern-cut embouchure, while I prefer the traditional). I haven’t even noticed the C# being flat on this one – in my case there’s no question that it was my inexperience.
And I wouldn’t call most tuning idiosyncrasies good or bad; they’re either what you’re used to or not. This is especially evident in the world of traversi – The separation between F and F# is generally pretty narrow, requiring a lot of lipping. Some flutes are optimized for the Fnat, some for the F#, and some split the difference. A particular player may have a strong preference for one over another, but it doesn’t make flutes built that way better.
From these reasoned standpoints to ‘couldn’t give me one of his flutes as a gift’, ‘go to a maker who knew what he or she was doing’ (which, by ‘took his advice’, you imply Dave didn’t) and ‘find a maker who can produce an in tune flute’… why so bitter now?
I’ve owned two of Dave C’s flutes and played others. He’s a very fine craftsman and a swell fellow, and the right person with whom to discuss such concerns. C# is often flat on simple system flutes, in my experience, but can be blown in tune. One method is to lift one’s chin and blow over the embouchure hole. It takes some practice but when you get the chops a lot of these problems can work themselves out.
Part of the reason, I think, why C# is usually flat on simple system flutes is that in order to get a “truer” C#, you need to vent the note by opening the Cnat key. Is that true, Jem, or others who know keyed simple flutes better than I?
On keyed simples system flutes - yes, the “standard” fingering for C# at the top of the 1st 8ve would use the long C natural key as an extra vent to bring the otherwise flat C# better into tune, and doing this is a habit easily acquired, even at some speed (see some of my YT videos!). The fingering would be (o,oo ooo,) or (o,oo oxx,). The availability of the C key to use in this way actually allows the C# tone hole to be made flatter to foster its other two uses - in cross-fingered C natural (versions of which remain necessary/useful in many contexts even when one has the key) and as the octave vent for D in the 2nd & 3rd 8ves. However, on a flute with no long C key, whether modern keyless or period flute with other keys but no C, the design compromises over the C# tone-hole have to take account of the need for a not-too-flat “open” C#.
On keyless flutes there is an inherent trade-off between the tuning of C# and C natural. Finding a balance between a not too flat C# and a not too sharp C natural (with useable, conventional fingering), is a challenge for all flute makers. As far as I know, there is no way to tune both notes perfectly. I have played a lot of different flutes from top makers, including Copley, Olwell, Murray, Lehart etc. They all make very similar choices in balancing the tuning of these two notes. The C# is always a bit flat in order to bring the C natural into range. I think some of the earlier comments implying that Dave Copley doesn’t know how to make a decent flute are way off base, and quite outrageous actually. In my opinion Dave’s flutes are among the very best for both tuning and overall playability.