Clotilde, Getting a clear shed with cards takes some practice. I've not done any card weaving for over a decade now, but when I did, I found that loosening the tension a bit while turning the cards, turning a little further than desired, then coming back to the desired position and raising the tension worked pretty consistently. Of course, for this technique to work, you need a way to adjust the tension dynamically (such as a backstrap or a foot-adjusted tension device). When I used a loom to hold the cards, I generally used slightly lower overall tension and did the overshoot-and-turn-back technique.
Kevin Karplus
