In this article, I will attempt to to explain the process of tuning a single reed duck call.
The best advice is to not take the call apart if it sounds good, there is no need in experimenting if you like the way it sounds. It is very rare that a reed will just go bad, and there are very few instances when one needs to be changed. If you see milky white spots or cracks in the reed then it is safe to say that you need to replace the reed, but most of the time when a call sounds flat or just doesn't sound new anymore; it is due to the cork being old and worn. What happens is the cork no longer applies the correct pressure on the reed and tone board, thus causing the reed to get sloppy. That being said, always change your cork or wedge first and see if this remedies the problem.
If a new cork did not solve the problem then a new reed will most likely be needed. You will need to make sure the bow of the reed is facing up on the tone board. When I say bow I am talking about how the reed bends when you place it between your fingers and squeeze it. The reed will bend either towards you or away from you and it should bend the same way every time. If it does not then get a new reed. When the reed is placed on the tone board with the bow up it would look like a "U". Make sure the reed bends up.
Now it is time to copy the old reed. You may do this by one of two ways. Lay the old reed on top of a new one and make sure it is aligned correctly, cut the new reed about a 1/16 of an inch longer than the old one. The other way is to lay the old reed on top of the new one and outline the edges with a magic marker.
Once the new reed is cut, install it onto the tone board and put the new cork in. If the new cork seems to tight then you can soften it up by chewing on it a bit. When the cork dries it will return to its original shape.
The difference in reed length is what effects the sound of the call. A longer stiffer reed takes more air to operate and is better on the top end; however getting the low subtle quacks out of it is harder. A shorter reed is harder to control and is not as good on the top end, but it is better on the low end. You have to find what works for you, and the way to do this is to cut small portions of the end of the reed. Take a bit off and try it and see if it operates the way you want it to, don't worry about how much rasp is in the call at this point, just get it to where you can comfortably control the call from top to bottom.
Once the reed is cut to length it is time to add some rasp to the call. This is simply done by "dog earring" the edges of the tip of the reed. Most people use a 45 degree angle on both cuts. The more dramatic and deep the angles are, the more rasp that will be in the call.
I hope this was helpful! Good luck and happy hunting!