Scooter at start, no train to the top after a short ride

Hey guys, you’ve been able to help me super well here, I hope you can support me with this problem again.The following problem: 1.) I had a piston eater and have successfully changed the cylinder including piston against a base Airsal 50ccm. 2.) When I start the scooter and start it, it has a great train and pulls up to its maximum speed (magic 53KmH). After the first brake it seems to me as if someone is slowing down the scooter under the roller. Up to about 30Kmh he pulls still great, from then on it is the purest cramp to bring him up. 3.) I tested already at the carburettor the stand gas screw and also the mixture screw with different adjustment possibilities, unfortunately the whole had not led to success. I am once again at a loss, it is really annoying to row around with such a cucumber on the road. Does one of you have an idea?