That’s really good information....
Let’s assume that vapor lock is the issue. If so, it would take some priming pulls to get fuel into the carb. The fuel in the tank would make its way through the filter, up the line to the carb, through the diaphragm and back out through the return line and back into the tank.
So it would take a few pulls to get the carb properly primed, etc.
Now let’s say the issue was negative tank pressure from a bad vent.
The same things would happen after you refilled the tank, fuel would travel to and from the carb in order to prime it.
So how would you know which scenario it actually was?
Well if it were a tank vent causing a vacuum issue inside the tank, you could hear air getting sucked in as you opened the cap. You might even feel resistance as you pull it.
If it were fuel boiling off to vapor I’d expect positive pressure when you opened the cap. The sucking or blowing do sound a bit different from each other.
Next time you open the tank before a warm start see if you can notice anything pressure or vacuum wise with the cap.