Liam you have a problem for sure. Your valve is currently doing the exact opposite of what it should when set up correctly.
I am almost certain this has happened because you’ve always stored the kart with the cover and compression spring on. The green below develops a “memory” from being in the closed position for extended periods.
You need a new bellow (the green rubber piece). Get one of these and rebuild the valve.
Go on YouTube and type in “rotax power valve” there are at least 4 videos showing you how the valve should work when set up correctly.
Also remember what I said about aligning the gasket with the little hole in the power valve housing. One other thing, make sure the curve in the blade faces downward when installed into the slot.
Let me know how it goes