Thanks for the ideas Philip, probably a stupid question (don't flame me too hard! a little bit is okay
) but is the fuel level in the bowl supposed to exceed the "level" marking? Quick Google reviles that the float may be stuck or an issue with the needle and/or seat, i am sending the carb away to get a kit put through it over the next few days so fingers crossed.
If it's just over the line then no problems but if the sight glass is full, then the carby is being flooded, so will make it cough and splutter.
Can you see any visible fuel leaks(weepage around joints) around the carby when you have the air filter assembly off?
If no, then I would definitely check my fuel pump before I did anything else. The old fuel pumps were rated at roughly 4 gallons per hour (18L/Hr converted is 3.3L/minute).
This is why the fuel line "in" fitting on the carb has a return line back to the tank.
All you have to do is take your fuel line off the carb, put it into a 2L container, and turn the ignition on. In 20 seconds you should roughly get around a litre of fuel in the bottle.