Hi Dave - Strika has hit the nail on the head I think ? if you have tried this carbie on all 3 pots and the problem follows it around.....it has to be the carb.
Last time I saw a problem like this it was the pilot jet. Looked clean enough when you held it up to a light source - you could see light through the jet - but it was still blocked. Needed a decent tub with carbie cleaner and then a good blast of compressed air.
How about the carb slide - is it in the right way around ? These are easy to drop in backwards.......
test for fuel flow by taking off the float bowl on the dud carb, fit it up normally then turn on the fuel - this is a good test for the needle & seat. If fuel flows in without issue - it has to be the low-speed circuits - idle mixture and pilot jet.
Does the choke actuate properly on this carb ? Need to suss this out as well, if this has an air-leak it could cause issues....
Good luck with it mate, let us know how you go.