it sound like you are sucking in too much air into the carb. is there
any kind of filter hooked or close to the carb?
If there is NO filter there than you need to call the moped store and
ask them if they have a air filter for your bike.
Another cause could be a fouled spark plug. see if the spark plug is
dark black on the bottom.
let me know how you make out!
Hope this helps,
14yr. Old Moped Mech.