Hi,
I noticed the same thing with my 501x wc....
The spring must be compressed totaly for it not to slip.
i thought the b44 spring would be a good replacement,...
but that one is way too big
.
I found the new spring in hong kong and put it on the car.
It is so much more direct!
here is the link;
http://shopping.rcmodel.hk/product_i...ducts_id=14800
good luck!
Miguel