The Z-S carb on late model Bs gets a lot of criticism, especially the water heated choke. Many people replace them, but I kept the original carb on my 1980 B for over 200,000 miles.
While stripping the engine down for a rebuild I discovered that the elbow and water passage at the back of the cylinder head that are part of the coolant flow for the choke were completely blocked.
When investigating issues with the choke it might be worth taking a look at this part of the coolant circuit, it's pretty easy to get at.