Have you considered the age of your original fuel tank flop tube?  Many are age hardened and don't really "flop" anymore.  New flop tubes from Aircraft Spruce don't fit the metric fuel tank fitting.  I have new, beautifully machined aluminum fittings that fit the original tank opening, (38mm), and new flop tube fittings.  See the Gallery; Users; John LaBarre, for photos of these fittings.
$75 plus shipping.  Supplies are limited.
(513) 227 7254