there's feature called "positive neutral finder" that uses centrifugal force on ball-bearings between 4th-gear and transmission shaft to prevent shifting into 2nd unless output-shaft & rear wheel is spinning. You may have assembled transmission in way that prevented these ball-bearings from relaxing back onto output shaft. Basically reversing function into "negative neutral finder".
CycleWorld - kawi positive neutral finder
Link to original page on YouTube.
Somewhere in this crazy Japanese guy's list of videos, he mistakenly believes his transmission is broken because it wouldn't shift out of neutral into 2nd while bike was off. So he tears it apart to find positive neutral finder feature. Really good presentation of mechanism. Can't find exact videos now, it's in his list somewhere
https://www.youtube.com/watch?v=yK1g6wyZ4Rw
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