This copy of the Vespa Fault Finding Flowchart is from the earlier Vespa Service Station manuals. It should help you identify and trace problems. Feel free to print it out and keep it in your glovebox, just in case you have problems on the side of the road. If you have a friend who is struggling diagnosing problems, why not send them the link, or print it out for them? It can work pretty well for Lambretta too.