Clean or replace the jets when in doubt.

Take a pic of the slide.  It sounds like it’s sticking from being worn/rough.  Take a pic so we can see how bad it looks.

If you flip the carb upside down, the slide won’t necessarily open completely.  The spring won’t compress enough for it to completely open.  If the slide won’t move at all when flipped upside down, it’s sticking.  It should at least move a little and clack around on the idle screw.  Be gently clacking it around so you don't break or damage the idle screw.  The spring compression shouldn’t be strong enough to hold the slide completely closed.

Note: The spring acts more to help close the slide after you’ve rolled off the throttle.  It offers a little resistance when rolling on the throttle.  Most of the slide action is vacuum-driven (thus CV carb).

Way to go!

Dang - tiny little #125 European main jet.  Talk about limiting performance to save a little savage smog!  A #145 is much better.  You'd probably really like a #150 or #152.5!  But going from a 125 to a 145 probably already seems like a major upgrade!

I live in Houston Texas so I ride right through the winter.  I don't know what winter storage is.    One of our Yankee members will have to answer that one.  Maybe Sta-Bil fuel storage additive (or that expensive Norwegian stuff you found)....  If I lived up north, I'd have to find a way to go VROOM VROOM in the garage for 5 minutes once a week to keep the fuel moving through the system.