Check the bottom surfaces of your mounted 10 micron fuel/water filters with a photo. If they look like this, it's really easy for them to have an air leak, even if you feel like you've screwed the filters on tight. The corrosion makes it difficult to gain an adequate seal.
I also had this issue and also went so far as to pull the deck plate and replace the 4 anti-siphon valves on the tank pickups. But these canisters is your primary opportunity for an air leak between the selectors and the primer bulbs, and it's where I was pulling air with motors of a similar vintage. Best of luck!
View attachment 7964
View attachment 7965
Feel free to reach out with any questions!