Tuning

Hi al
I have a Martin CK1 which I am happy with but the F on the C string does not ring out. I have tried D tuning and it makes the thud still on the F I
have tuned to Eb and got the F to the 3rd fret. My question is how far can I tune it up and maybe lose the F?
Thanks Phil.
P.S. I have changed the strings problem still remains.

You might check out the "Dead Notes" thread. Sounds like a similar issue?

Yep, sounds like dead note. Here's my recent thread on the subject:

http://forum.ukuleleunderground.com/showthread.php?133107-Talk-to-Me-about-Dead-Notes