The spring is there to maintain pressure on the needle when a bumpy road makes the floats bounce up and down.

If the height is set correctly and the needle / seat surfaces are good they should seal. If not, then something is broken - holed float, broken spring, dirt...
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball