how can i fixaprilla sr50 scooter ditech?

can any one help just had new barrel and piston fitted and new sport exhaust.starts no problem runs no problem.after first start up and run pulls away all the way to 56mph no problem but when you come to a stop and then pull away very sluggish to 30 mph after that goes like atrain from 30 mph to 56 mph no problem does any body have an idea what the problem is would be very great full thanks