If your choke isnt moving in detents, then its wrongly setup, the pulling it out about 10% should engage the fast idle,
You can check at caburetor, then pull the lever at the carb by hand and you will see the choke plate close. This is the second detent, if your cable is setup wrong or your choke levers are bent or loose, the choke wont work as it should,
speaking of the sealing area of the air filter pan, put some petroleum jelly on the air filter element top sealing part and tighten the lid, then remove it, the petroleum jelly should be seen on the top cover too, this is one of the most important seals, the second is the rubber seal that sits on the carburetor mouth, absolutely no air leaks should exist in these two along with the pcv hose and fresh air tube for the pcv otherwise your carb will never work correct.
The lid outer seal is just a soft foam seal to prevent wear and rattle and forces the air to be sucked from the snorkel and not the edges,
Before going to mechanic and carb guy again and again, check everything yourself for actual working, correct installation and finally proper operation. It seems like most of these mechanics dont know how to setup a carb anymore.
Btw, have you checked if your distributor is working perfectly? Is the centrifugal advance system free and snappy? (Its not the vacuum advance) - the centrifugal advance is the spring loaded advance system inside that advances timing relative to rpm. A lazy distributor will manifest its symptoms as a dirty carburetor too.