gy6 50cc no power...

[padly1995]

Hello, im repairing Jonway gy6 scooter for some guy.
First it was stuttering and i founded out that there was white plastic circle part that goes over the needle missing in the carburator.
I installed new one and scooter has stopped stuttering. But it still doesnt have any power. It goes around 20 - 30kph, it has realy bad acceleration and it goes on realy low rpm.
If I put scooter on the center stand, so wheel is up, it revs normaly, however if i press brake it instantly loses power.
I tried to play with air intake, but there is no difference. I checked valve cleareces and they are both OK. Also the camshaft is lined up as it should.
I can also hear some strange noise comming from under the flywheel I think.
What do you suggest me to do? Do you thing this could be CVT problem?
Thanks