Can you post a picture of the carburetor flange where the air cleaner mounts on it, and the matching gaskets and insulator please Paul? Remember, the air cleaner side of the carburetor, not the cylinder head side.
I'll tell you where I think this may be heading. On many of the Keihin carburetors Honda uses, and the Ruixing and other copies of them used on chondas, the float bowl vent runs to the air cleaner through a port in that flange. If you put the gasket or insulator on backwards, you block the float bowl vent, and probably get a result something like what is happening.
