The backfires cacan sure come from the blown out exhaust gasket. The leak allows fresh oxygen in the system and it combined with the heat fires off. Get the manifold fixed before going any further. You most likely need a carb rebuild, the surging souunds like a bit of overfueling. Check the sparkplug color. The overfueling could have caused the first backfire that caused the exhaust leak! A lean mixture can also cause a surge, junk in the carb may be blocking a fuel port, so again, a rebuild is needed. Joe
