Was the carb needle on the 1st notch (towards the blunt end?)
Simon is correct , if it goes on the middle slot – can it help ? as that’s OEM. Hence its towards the 3rd tip towards pointy tip, it means you get to “mains” a bit sooner,from stoodstill “pilot” idling.
The reason for the 3rd groove towards the tip is when the slides lift , you get a bit of enrichment since the needle is up 2mm ish out of the carb, so that tends to affect the 4-7k rpm flatness (mid-range) but the 125cc usually revs past 7k pretty quickly on hard throttle when the bike is okay & the main jets simply just take over at this rate.
If the mixtures have been messed with , floats messed with (like Cobra2 says on wrong float heights) = it can throw things off a little bit.
Also when you change gears , do you get any jerk-jerk-jerk behavior then it revs up / rides normally ?
Also can you supply pictures of your plugs so we see what’s happening with the mixture ?
Any intake mods ? any exhaust mods ?
Lots of questions i know but take ya time, more info is good for our heads