WH Lithuania wrote: I have no idea what is Z.
Most foil kites, including the Joker, have 4 rows of bridle attachments to the canopy. The row closest to the leading edge is A, then B, C, and instead of D most companies call the last row Z. Z applies the most tension to the trailing edge and is usually in a 1:1 ratio with A in terms of how the pulley systems work.
When Z is tight, it can affect performance by increasing lift at slower speeds, but the price you pay is often a loss of collapse stability and less upwind angle. Loosening Z might take some grunt out of the kite but will make it ligher bar pressure and a bit easier to fly.
The Joker usually comes with 3 knots on the Z line so you can adjust. Try the middle or the loosest knot if not already there. If you're on the loosest, you can tie a new knot further up to add some slack to Z, just make sure you get the left and right side mostly the same so it flies straight.
Z can also be tricky because it does not carry a ton of load while riding, so it's prone to line shrink, so even if you're on the loosest Z setting, it might be tighter than it was on the kite's birthday, so if it's a used kite I'd give it a double check as it may be a bit shrunken and thus the source of your troubles.