That's a good question Richard, and I await other answers.
I was of the belief that raw fuel didn't flow into the inlet but escaped out of the tickler when the fuel level got high enough (just below the venturi) so that the first few sucks got a really rich mixture, and stayed rich till the fuel level dropped to normal.
We will need to remove the air filter (if fitted

) and with a torch and mirror have a look. A carb on the bench with fuel hose connected would be easier.