Griffin-pro ball diff issue

[mr. ed]

While assembling my griffin pro I found 2 springs in the same bag: a silver one and a black one with no other apparent difference than the colour.

One 's for the slipper the other for the ball diff, but there's no mention of what to use where in the manual.
Going from some pics on the side of the box I dedcided to use the black spring in the diff and keep the silver one for the slipper.

Unfortunately I can't get the diff to run smooth and not slip. Even when the diff is pretty tight you can turn tcenter gear easily by hand while blocking the outdrives. Did I pick the wrong spring? OR is this just a common problem with the model and should I go for the geardiff of the standard griffin and RT sport?

I've already taken it to a couple friends/racers to confirm I built correctly, and have built several different ball diffs before without ever seeing this cars problem. My theory is: the diffplates are not stiff enough???