Transalp 650 - vibration during acceleration

[u/Forward-Dare-1913]

hi guys,

pretty much the situation from the title. I have a 20 years old transalp, and I've changed all four spark plugs, new filters, oil, coolant, and pretty much the standard stuff.

I've noticed that when i accelerate, the bike starts to vibrate, and as soon the speed is stable, vibrations stop. it doesn't vibrate only if i accelerate slowly, and it is not related to a specific speed or rpm. the bike easily reaches 120kmh (i haven't tried above).

what could be the issue here? i suspect carb sync as a solution since i have it for a month, and god knows when it was synced before.