Rotax junior holds back then at 12k rpm has metallic howl

I have a pre evo max junior, has just developed a strange problem, it’s down on power like its holding back then if you push it through to 12k it has this loud metallic howl , i only came on once the kart had done a few laps and when i got home and kart was cool it was revving up nice . Hope someone has had this problem as i don’t know where to start.

Update, the stinger in the exhaust was broken and closed the exhaust.


How did you find it? By chance or something or someone pointed you that direction?

Was a suspicion I thought air filter first but was ok , then I remembered an old post on karting 1 about needing to change the wool or it will burn the stinger and block exhaust