You just have to check things first.Things like making sure the air filter is clean and not blocked.It is harder to diagnose when not in front of me,but the first I'd check is carby make sure it is clean and the needle and set are in good working order.Make sure the cut off hasn't worm through it's rubber boot and shorting out.Also check the fuel cap there should be a small hole make sure that's not blocked or it will get vapour lock and that will stop it.So check cap first then the carby.
That's were I'd start looking first.
Here what they have inside them.
[Linked Image]
Hope this helps.

Here for a good time,not a long time.