An overly bitter and sometimes even burned taste can occur when coffee is over-extracted. The most common reason this happens is that the coffee grind is too fine. Under-extracted coffee is often the cause of a weak and flavorless brew. Under-extraction can occur if your coffee is ground too coarsely and/or unevenly.
For the Moccamaster, we recommend a medium-coarse grind, which should look like coarse sand or sea salt (see image below). If having your coffee ground, use these descriptors to ensure you get optimal coarseness. Note that most pre-ground, bagged coffee tends to be too finely ground for ideal operation.
For even more about grind size, read our blog article The Importance of Coffee Grind Size.