D id this start as soon as you picked the moped up. How was it behaving before. If it was just serviced I'll assume that it has a new plug, fresh gas and a clean carburetor. It sounds as though it may have a vacuum leak. We see a lot of Tomos mopeds that have the A-35 engine come in without the stepped rubber intake seal between the carburetor and intake manifold. Also see them come in with the seal backwards. Both will cause simmilar symptoms to what you are describing. In cold weather the symptom is worse because the air is denser when it is cold and the leak will appear to be larger and cause it to run even leaner. Try spraying around the front of the carburetor with carb cleaner while it is idling and see if has any eccect. Might want to spray around the base gasket and reedvalve while your at it.