Check
that the selector spring is in good state and fitted in the right sense (common
mistake).
Correct
sense for the spring.
Check
that the washers and the o-ring are fitted correctly (spring-washer-oring-washer).
Fit
temporarily the selector, select the gearbox in 4th (not neutral as advised in
some manuals) and check that the travel is the same on top and bottom.
If
necessary, adjust with the excentric screw.
Fitting
a o-ring (non-original part) on the selector case helps to eliminate any oil leak
if the axle is used.