Hello Andrew,
There should be a black hose going from the back of the carby to the vacuum nozzle on the cast iron block.
Poke a piece of wire like a paper clip through its hole, to make sure it is clear.
Also, if the diaphragm is sealed properly, you should not be able to suck or blow through the black tube connecting to the back of the carby. (diaphragm end) This is a test you can do.
The flywheels fan will create a vacuum through this hole and connecting tube & on the back of the diaphragm, this fights against spring pressure in the carby, whch makes the poppet valve open & close to give more or less fuel depending on load.
Hope this helps.
