Ktm sx 250 ccm bj 1990 stutters at full gas

Hello together. I got a ktm sx 250ccm. I’ve already driven a few laps and had to find out that I can’t run out of gear properly, because the engine starts stuttering at higher speed and has no power. I have now set the nozzle needle a level leaner but can’t detect any improvement. Did you get a few tips or vll also have the same experiences and can help me?