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Sour espresso is easiest to fix when you first decide whether it is underdeveloped or simply bright. A coffee with lemon or berry acidity can still be sweet and satisfying. A shot that is sharp, thin, and abruptly empty usually deserves a closer look at extraction. Start with a stirred, slightly cooled sip rather than judging the first scorching drops.

Check whether the shot is repeatable

Write down the dose, beverage weight, grind setting, and shot time. Keep the timer convention consistent, especially if your machine has preinfusion. An 18-gram dose and a 36-gram beverage can be an initial reference if that dose fits the basket.

Look for major variation between shots. If one chokes and the next rushes at the same setting, do not immediately chase sourness with a finer grind. Check basket dryness, distribution, retained grounds, and whether the dose really stayed the same.

A bottomless portafilter can reveal spraying, but it is optional. Taste and repeatability remain useful even when you cannot see the basket's underside.

Confirm that the machine is ready

Follow the manufacturer's warmup procedure with the portafilter in place where appropriate. A cold basket or group can pull heat from a small shot. The display setting is not necessarily the temperature at the puck.

Use the machine's recommended flush routine rather than a generic long flush. On some designs, unnecessary flushing can create the very temperature inconsistency you are trying to remove.

If the machine offers brew-temperature adjustment, save it until dose, flow, and yield are stable. If it does not, you can still make meaningful changes without defeating the thermostat.

Extend an evenly flowing shot

For a sour but otherwise orderly shot, compare a slightly longer beverage yield. With 18 grams in, move from 36 grams out to about 40 or 42 grams while leaving the grind unchanged.

The longer shot usually becomes less concentrated, so do not judge it only by thickness. Look for increased sweetness and a more complete finish. If it becomes thin and bitter without losing the sour edge, uneven extraction may still be present.

A longer yield is particularly worth exploring with some light roasts, but it is not a command to pull every coffee long. Keep the version that tastes better.

Go finer when fast flow supports that choice

If the shot reaches the target weight unusually quickly and tastes sour and weak, make one small grind adjustment finer. Keep dose and yield fixed for that comparison. Follow the grinder's instructions when adjusting, especially toward finer settings.

Stop moving finer if the shot stalls, sprays, or becomes less predictable. Excessive resistance can encourage uneven paths through the puck. Improve distribution and basket fit instead of compensating with an ever finer grind.

Do not tamp harder to imitate a grind change. A level, consistent tamp is a preparation step, not a reliable flow-control dial.

Consider the coffee before exhausting the bag

A very fresh coffee may behave differently after resting. Follow the roaster's guidance and compare over several days if gas release is disruptive. Conversely, stale beans can produce a hollow cup that more extraction will not make lively.

If a well-extracted shot remains more acidic than you enjoy, choose beans with a different roast or flavor profile. Taste preferences do not need to be corrected by forcing every coffee into the same style.

Record the best dose, yield, and setting, then stop. A useful repair gives you tomorrow's recipe, not an endless sequence of increasingly complicated changes.