Why does my coffee taste burnt or smoky?

A burnt, smoky, or ashy taste comes from one of two places. Either the beans were roasted very dark, so they already taste of char and smoke before you brew, or you are over-extracting by brewing too hot, too long, or too fine, which drags out the harsh compounds that show up last. The good news is both are easy to test and fix at home.

Is it the roast or the brew?

Smell and look at the beans first. If they are very dark, almost black, and shiny with oil on the surface, the roast is doing the talking. Dark roasts are pushed past second-crack, where sugars stop caramelizing and start to carbonize, and that is what reads as smoky or burnt. No brewing technique fully removes that; it is baked in. The fix is to switch to a lighter roast. A medium roast will taste sweeter and rounder, with the smoke gone.

If the beans look medium and smell fine, the problem is most likely your brewing. That points to over-extraction.

Fix the brew, one change at a time

1. Cool the water. Off-the-boil water around 90 to 93°C (194 to 199°F) is plenty for most coffee, and darker roasts in particular want it cooler, closer to 88 to 90°C (190 to 194°F). Boiling water scorches the grounds and pulls out bitterness. See water-temperature-by-roast.
2. Grind a little coarser. Finer grounds extract faster and harder. Coarsening up slows the extraction and softens that burnt edge. Use the grind-size-guide.
3. Shorten the brew. Less contact time means less of the harsh, late-extracting flavor. Pour a touch quicker, or pull the plunger or filter sooner.

Burnt and ashy live at the over-extracted end of the sour vs bitter axis. If you overcorrect and the cup turns thin and sour, you have gone too far the other way: nudge the grind back finer.

If you cool the water, coarsen the grind, and shorten the brew and it is still smoky, stop blaming your technique. The roast is the cause, and a lighter bag is the answer.