Why is my espresso shot running too fast?

Shots running too fast (under 20 seconds) produce weak, sour, under-extracted espresso. Causes: grind too coarse, dose too light, basket not properly seated, or a worn group head gasket allowing pressure bypass. Fix: grind finer first — this is the most common cause. If you've already grinded very fine and the shot is still fast, check that you're using enough coffee for your basket size (most double baskets need 17–20g) and that the portafilter is locked in firmly.