Have you run a proper compression check? Better yet, a leak-down test if you can find someone with the right tool.
I'd start with a careful tuneup, as others have suggested, and check everything extra-carefully as you go. Set the advance at least as much as stock, and more if your ear says it's not pinging. In general, the more advance the better the mileage.
Fuel leaks, too. Watch the carb area while you drive it. You can sprinkle talcum powder around the carb and manifold and any gas leaks will mark it. A bit messy but a good leak-finder. (And talcum is way soft - a bit in the engine won't hurt, but don't sprinkle it in.)
George
