Thank you so much, that makes so much sense.
I think you may possibly have solved our problem. My husband will be taking a look at it tomorrow.
Everything on our moped is super-clean, & well maintained, but the gas line is old, and I think there's one area where it's kind of squeezed against a bolt. We'll be replacing that hose just to be on the safe side as well as checking the petcock for any obstructions.