A few thoughts.
The surging sounds like an air leak. I've had five FA50s and never had an air leak. Check it with the engine running and spray around the engine with carb cleaner. If it makes it stall there's an air leak there.
Check the carb. In the photo the bolt does not look tight (may be a illusion).
Also, the fuel cock is on reserve which will suck the fuel from the sump in the tank through the reserve fuel line. Could be setiment in the sump. Make sure fuel cock is on run.