If you are running with the stock exhaust manifold, there is a port between the 3 and 4 outlets that warms the air to the choke pull off spring. It constantly draws air through the port, even after the choke is fully open. Typically, the space between the two ports burns through.
Not saying you should but if you did take off the exhaust manifold, you would probably find a large hole burned into the riser space. Once this opens, it only gets larger because of the extreme temperature. When this happens, you are sucking exhaust gas into the choke riser tube and into the spring chamber at the carb. This space fills with carbon and corrosive material that rusts the spring or gums up the rod movement.
There are a couple fixes. One takes a fair amount of money and uses a new or intact salvage replacement exhaust manifold. The other is to remove the choke riser from the manifold hole and plug the holes top and bottom. I tapped them and used a couple screws. Then you clean out the spring chamber and clean up the rods real well to where everything moves freely. Then, you get an aftermarket stove and riser tube that does the same thing but does not suck in the exhaust. You can skip the little stove/collector and just wind copper or aluminum tubing around the header somewhere. It just needs to heat the air. I liked the look of a manufactured item.