Okay, you've tweaked grind, time, and ratio, and it's still sour. What's left?

• Water quality. If your tap water is hard or high in minerals, it can mess with extraction. Try filtered or bottled water—it's a cheap experiment.
• Brewer cleanliness. Old coffee oils build up and go rancid, adding off flavors that mimic sourness. Give your siphon a thorough clean with a mild detergent and rinse well.
• Bean freshness. Even if your beans smell okay, they might be past their prime. Check the roast date. If it's been more than a few weeks, get a fresher bag.

If none of this helps, your beans might just not be suited to siphon brewing. Some roasts are naturally more acidic, and siphon tends to highlight that. Try a different origin or roast level.