Hey guys, new here
I was looking at the forum and I saw so many competent people around here so I subscribed and hopefully someone can help me ^^
I own an XTZ660 Tenere Yamaha from 1992 with 12000km (aside this bike I own some more ones like TW200, TW125)

I have a very weird issue while shifting, let me explain carefully ok?
I checked with various mechanics, they all say the shift box is awesome (one of these mechanics is also a very great friend of my dad and he modify bikes for race tracks, tournaments etc completely trust him) another mechanic also very good friend with years of experience in the field confirmed that the shift box after test ride my bike is very clean and issue-less
One of the mechanics suggested to change front and rear sprockets, chain, and flexible couplings into the rear wheel (I don't know if that's the correct english name, basically the rubber things that absorbs abrupt use of the throttle, shifting etc etc) because probably the issue I am having is most likely caused by that, and I can believe him since the chain when I purchased the bike was basically rusted, and with a lot of missing o-rings in the chain
Everything is brand new, changed oil with Motul 5100, changed oil filter, air filter, set correctly by service manual the clutch free play, drive chain slack properly set, chain of course lubed and taken care meticolously

Now here is the problem, note that it's ONLY speed related, You'll get what I mean in a sec

It only happen when going from 1st Gear to 2nd Gear, so only shifting up and only from 1 to 2

If I shift gear when I am kinda slow the gear shifts in no problem, smooth, buttery, very delicate push with foot

If I try to go faster, like 30-40kmh in 1st gear and I try to shift, well... It will 75% of the times go into neutral, or you hear a bad clunk while passing through Neutral and go into 2nd, or you hear ta-ta-ta-ta-ta-ta and you can feel the shift lever "banging" on your foot slightly, and then the gear will enter

Now you are thinking: RPM too high, wrong

If I go slow, pull the clutch, accellerate with clutch pulled to increase the RPM to like 6k (redline is 7 when the ignition will then be cut off) I can still shift without problems
Viceversa if I accellerate very fast, or go very fast in 1st gear, I pull the clutch, and maybe wait 3-4 seconds withotu doing anything, then try to shift, it will clunk, tatatatatatata etc etc, seems that the issue is purely speed related

I don't know what the hell I have to try I really do not want to dismount the engine since I already have the other bikes dismounted (I am modifying a couple of them and another is being transformed into something else, Engine for a kart probably) so that's my only my of moving around, and with job, girlfriend and many other stuffs is something I really can't do...

Note that all the other gears are absolutely great, and even shifting from 2nd to 1st is great, no issue, foot lever is buttery smooth, just this horrible going up from 1st to 2nd is bugging me a lot and a couple of times put me in horrible dangerous situations (overcoming a car, shifting, can't shift properly and I find myself in the middle of the road with other cars coming from opposite direction, horrible...)

The rest of the bike is absolutely fine, engine is great, is has no noise at all, it reached top speed in no time and can mantain it in any situation (160kmh while yamaha claims the bike can do max 165)

A small "trick" I am using to help minimize this issue is preloading the lever while on first, I preload with left foot (actually preload is gently, I litterally pull hard on the lever with the foot) so the moment I close throttle and pull clutch it makes a bit easier for the gear to enter, but this seems so bad and not correct to me, all the other bikes I have driven, even older than this one and with tons more KMs on the engine and with VERY abused clutches and gearbox showed litterally no symptoms or problems

Recently I tried to lower my idle speed via the screw near the carburetor and actually the problem is getting super better compared to before, I can shift most of the time without any issue and onyl sometime with a lil bit of noise, but no grinding or anything problematic at all
VERY VERY few times happen that I pull the lever to change gear but I happen to be stuck in neutral (It happens generally when I shift very slowly, and I mean very slowly because I am lazy or talking with my passenger haha)