Thanks once again. I have replaced all the diaphragms and gaskets in the carb along with the needle assembly. Sprayed carb cleaner everywhere down every visible hole while doing this and so am fairly sure it's as clean as it's going to be. So to draw the fuel up the feed pipe I need to run the engine (for 5 seconds at a time dropping in fuel via the air intake port) with the choke fully on? Also, am awaiting new gasket material so I can remake the last gasket from the carb to the engine body as the existing one is a bit manky. If the impulse port is being blocked as I reassemble the engine (due to lack of positive location of the gasket) then I guess there'll be no pumping action at the diaphragm.
I wish I had not seen this reply
For quite some time the check valves fitted to cube carbs have a rubber that is badly affected by carb cleaner
Slide the carb off the mounting bolts but leave the fuel lines attached.
Blow through the carb air hole holding the throttle wide open
You should get a strong flow of atomised fuel blowing out the engine side if the carb is working as it should
And this will happen even if the fuel lines were originally empty as the high speed air will suck fuel right out of the fuel tank .