Definitely a common problem on the older ones. What happens is the plastic that slides down into the aluminum carburetor starts to swell up as it gets older and it sticks in the holes. It sits in a recessed hole in the bottom and of course around the top where it slides through and both areas get kind of fat and prevent the plastic from flipping back and forth with a spring pressure. Just take the air filter housing off pull the brass choke plate straight out from the front with needle-nose pliers then you can lift the choke rod right up. Be careful with the linkage or take that off first. I use some medium grit sandpaper to sand down the plastic at the bottom and around the top where it will fit into the hole. Once you test it and it has plenty of clearance to spend freely lube it with some spray grease and put it back on and tension the spring the proper turn around. This always solves the problem of sticking chokes for me.