google moped riders association, and check motobecane users and service manuals. Might be links on wiki to general moped stuff. If you can find the service manuals on pdf of the motobecane and other mopeds, they pretty much tell you what to do.
Tip , If you do not have in line paper element filter, install, at least 1 and 2 is better. These old bikes have dirty and rusty gas tanks.