Peugeot Speedfight 2 won't stay idle?

Hi! I have a Speedfight 2 and I had a problem with it. I needed to replace the cylinder.
I did it with a friend and everything was okay but after we assembled my scooter it just didn't want to stay idle.
I can start the scooter but it cuts out in a few seconds.
With trying, I can start it basicly in 5-10 minutes by pulling the gas why i start.

I tried the idle screw, I checked the spark, the carburetor is fine as well i think.
What can be the problem?

Balazs
May 2012