As i saw photos the problem is from piston quality. Usually Mitaka cylinder is ok but pistons coming together are crap. If it's this type that's one of the reason .My mito evo 96 has mikuni tm35, and standard pilot jet there is size 45, however it was running insanely rich - even with air screw fully out. Trouble starting, completely wet spark plug, juices coming out of the exhaust.
The cylinder I had at the time (now seized and destroyed, oops) was unmarked mitaka(i guess it is a0401, will try to check today), but from the factory the bike had 73037 cylinder(at least the manual says 130 transfer and 190 exhaust). Everything else afaik is standard from the factory.
So I got some jets, and re-jetted it with size 20 pilot and upped main jet from 310 to 350(standard mikuni jetting for tm35), to keep the total fuel flow unchanged. Aaand it seized after running wide open throttle for quite a bit and closing the throttle fully before a turn.
I suspect that this happened because I was running too lean pilot jet, causing detonation (there is a small molten spot on the piston). Contributing factor might be tight clearance between piston and cylinder (yes, rookie mistake to assume that cylinder kit comes with matched piston).
I get the idea behind rich pilot, it is exactly for extra cooling when going from full throttle to closed throttle, however now I dunno if it was designed to idle like crap.
Now I am thinking that port timings (124 degrees for a0401 and 130 for 73037) affect air velocities - same amount of air is to be delivered in a shorter time - increasing air flow through the carb.
I will get my old 73037 replated and new vertex piston, see how it goes.
Why would cagiva use such a rich pilot jet, that can not (in my case) be leaned out with air screw? What might be the reason for this rich situation?
Pics here: https://photos.app.goo.gl/7rWuxXodtZir6F4y7
Also small pilot can do this but it needs time and not at the time you are closing throttle next minute seize.
Always companies tune richer all high performance 2 strokes bikes, just for their engines to be accurate and have longability before failure.
Btw check carb specially air chocke, if when is close has small flow then it will be like you have rich mixture at low throttle opening or idle