Hi everyone,
I managed to rewind the spring successfully and put everything back together again. Unfortunately the problem is not resolved, and I realised the problem might not be this spring.
@phass:
The problem with my camera is: The upper door close fully with good tension, however the lower door does not close fully (even though when you push it, there's good tension behind). I think it has something to do with what the gap between the two door that you talked about, rather than the spring. When I turn the knob (on top of the gear for the spring), both door moves together however there's always 1cm gap between them. And when the upper close close, the lower door stops, 1cm away from being fully close. When I turn the knob further, the lower would close.
I tried to hold the small shiny disc you described to adjust the door, however there seems quite a lot of resistant doing so and I dont want to force it. Any further advice for me please?
For anyone who is interested in the original problem, if your auxiliary shutters are jammed or sluggish and without good tension, I encourage to attempt rewind the spring. It can be quite fiddly however not overly technical to do. I did it before reading Phass method (put the outer hook in place first), and I put the central hook in place first. I used two fingers on one hand to hold part of the spring that has been winded into the gear, and use the other hand to wind the outer hook around it. Its quite fiddly however there's not many things that can go wrong. If it jumps out you can just try again. It took me a few tries. Good luck.