It seems straightforward now, Paul. I went off in a different direction in the belief that it worked properly without the entire air cleaner apparatus, but badly if any part of the air cleaner were fitted. However if it runs badly regardless of the air cleaner fitment, and gets better when you partly close the choke or cut off the air bleed, we are down to two main suspects, both of them pretty routine, in happening and in repairing. It sounds as if it either has a leaky carburetor insulator gasket (they often split when the carburetor is removed, and will not work properly thereafter) or an obstruction or misassembly in the carburetor. I suggest you begin by removing the carburetor and inspecting both insulator gaskets (there is one on each side of the plastic insulator). If they are perfect, you need to look primarily at the main jet and the emulsifier. All these things are very easy to put right.
