Why does my Moka pot sometimes produce GREAT coffee and sometimes awful coffee? So I try to grind about the same each time but for some reason if I don't hit the jackpot on adjusting my hand grinder I get sour or bitter coffee. Why is my Moka Express so finicky?
Actually, this question is self explanatory. I assume you only need some guidance and confirmation for what's going wrong.

A better known fact is:


if your coffee is underextracted, it tastes sour.
if your coffee is overextracted, it tastes bitter.


Now, let's tie up this to grind size:


if your grounds are finer, your coffee overextracts if other conditions are kept stable.
if your grounds are coarser, your coffee underextracts if other conditions are kept stable.


It's known that Moka pots are not controllable devices. So, all the other conditions (temperature, pressure, etc.) are nearly same each time. Thus, it's obvious that your grinding directly affects the taste of your cup. You should be consistent about your grounds. :)

And a final note, one of the jewels of this site is this extraction chart that you may check.