Took the advice to clean the automatic choke linkage. Problem was that the spring-return lever was very stiff. I used brake parts cleaner sprayed onto the swivel point between the black plastic body and the metal lever. Exercised it a few times and continued to apply cleaner till it worked freely. Oiled with light machine oil.
When you take the carburetor off, don't pull the bolts free. That way all the parts will stay aligned. Only the motor-to-choke gasket and, of course, the black plastic choke body need to be separated from the carburetor. The gasket goes back on with the square corner down and towards the front of the mower. Mower started on 3rd pull after it would not start after about 15 pulls before.