I don't normally use pressure so correct me if I am wrong but I think the fine-thread needle is for higher bladder pressure so perhaps with backplate pressure you should be using the standard needle valve.

Also, after squirting some fuel through the needle valve assembly, check that all three holes are clear on the venturi.

Make sure you do not tighten down the venturi too much or you will break the carb body.

Good luck,
George