If your coffee still tastes sour after the quick check, it's time to adjust your grind. This is the single most effective dial.

• Go finer, not coarser. Sour means under-extraction, so you need more surface area. Grind one step finer than you normally would for siphon.
• Brew shorter. Sounds counterintuitive, but a shorter brew time can actually help if your water is running extra hot. The longer the water sits on the grounds, the more it extracts—but if it's too hot, it extracts harsh acids first. Try cutting your steep time by 15-20 seconds.
• Add a few more grams of coffee. Think of it as insurance. More coffee buffers against sourness, giving you sweetness to work with.

Make one change at a time and taste after each. You're looking for that sweet spot where the coffee tastes round and balanced, not sharp.