Most people own a grinder, a scale and a kettle, and still pull sour shots because one variable is out of range. Here is the whole diagnosis in one table, then the reasoning behind each row.
The two-minute diagnosis
Taste first, then change one thing. Espresso gives a very consistent signal: sour means under-extracted, bitter means over-extracted, and both at once means the puck is channelling.
| What you taste | What it means | First move | Typical adjustment |
|---|---|---|---|
| Sour, thin, no sweetness | Under-extracted | Grind finer, or raise the dose | +2 g dose or one step finer |
| Harsh, ashy, drying finish | Over-extracted | Grind coarser, or drop temperature | -1 to -2 C |
| Sour and bitter together | Channelling | Fix puck prep before the grinder | WDT, level, tamp straight |
| Weak but bitter | Too much water for the dose | Shorten the yield | Cut ratio from 1:2.5 to 1:2 |
| Gushes in 15 seconds | Grind too coarse for the basket | Grind finer until 25-30 s | One step at a time |
| Drips in 45 seconds | Grind too fine, or dose too high | Grind coarser or lower dose | -1 g dose |
| Inconsistent shot to shot | Retention or temperature instability | Purge 2-3 g before each shot | Weigh the purge |
Start with dose, not grind
Pick a dose your basket actually holds. An 18 g basket wants 18 g, maybe 17 to 19. If you dose 20 g into an 18 g basket, the puck touches the shower screen and the shot channels no matter how good your grinder is.
Once the dose is fixed, ratio sets the strength and grind sets the time. Changing dose and grind at the same time is how people spend a whole bag of beans going in circles.
Then set the yield
Weigh the output in grams, not millilitres. Crema is mostly gas, so a 36 g shot can look like 60 ml in the cup. A 1:2 ratio from 18 g in gives 36 g out, the modern default for medium roasts.
Light roasts usually want a longer ratio, 1:2.3 to 1:2.5, because they are denser and release solids more slowly. Dark roasts want 1:1.8 to 1:2, because they extract fast and turn ashy beyond that.
| Roast level | Ratio (dose:yield) | Why |
|---|---|---|
| Very dark | 1:1.5 to 1:1.8 | Extracts fast, bitter early |
| Dark | 1:1.8 to 1:2 | Classic Italian feel |
| Medium | 1:2 to 1:2.2 | Modern cafe default |
| Medium-light | 1:2.2 to 1:2.4 | More clarity, less body |
| Very light | 1:2.4 to 1:2.6 | Needs the extra water |
Then grind
Adjust grind to hit a time. For an 18 g dose at 1:2, aim for 25 to 32 seconds from first drip. Faster than 22 seconds and the puck is under-resisted; slower than 38 seconds and you are close to choking the machine.
Move one grinder step at a time and pull a shot after each change. On a stepless grinder a 5 micron move is a real change. On a stepped grinder one click can be too much, which is why stepless is worth the money.
Temperature comes third
Temperature matters most for light roasts, where 94 to 96 C pulls out sweetness that 90 C leaves behind. For dark roasts, 88 to 92 C avoids the burnt edge. If your machine has no PID, temperature is probably fluctuating 3 to 5 C, which is why shot quality wanders.
The three-shot rule
A grinder keeps 1 to 4 g of the previous dose. After changing the setting, the first shot is a blend of old and new grind. Pull two and adjust on the third. This single habit removes most of the frustration from dialing in.
Keep a log
Dose, yield, time, grind setting, temperature, taste. Six numbers. After twenty shots you will see the pattern in your own setup, which no guide can give you.