hi everybody

bike is 2002 model, used to run without big issues during the years.
mileage is over 160k kms.

problems started couple weeks ago, pretty sharp and out of nowhere
during the day-long road ride - suddenly felt bit weird and weak
feedback from gearshift pedal.
during / after stopping discovered that switching into gears became
tricky - mostly, in these cases: N > 1, N > 2, 1 <> 2 and 4 > 5.

"tricky" means that sometimes gear changing was not happening at all
and i had to roll the bike a bit, or turn off the engine and then attempt
to change gears. also, sometimes after shifting it would shift back (by
itself!) in previous gear pretty soon.
occasional false neutrals also used to happen.

some further ride showed that this problems used to happen roughly in
the half of the shifting attempts in the above listed cases, while
2<>3<>4 cases were mostly ok, which let me get back to my place.
some other attempts on next days showed that switching was happening
much better when engine was still cold.

before removing clutch cover, was hoping for some wear on the starry
end of selector drum or on the ratcheting mechanism of shift shaft.
but no.
no evident signs of wear.
following 0,04K videos show how the selector drum gets stuck somewhere
between the positions and failing to properly engage gears.






what happens when selector is stuck? no change unless some input from
rear wheel or engine.

as a last hope, fitted another stopper lever with much bigger roller
(spare from other model), but still no success.




seems i'm facing some major problem with unknown extent, involving
splitting the case and then who knows..

are there any clues what to expect? or what might be the reason?