It sounds like the transition slot is running out of fuel before the mains are fully flowing. Before you pull the carb apart you could try restricting the idle air bleeds with some small gauge wire and see if it improves. This will richen up the idle/transition. If it helps you know the problem is the ifr/iab relationship.
If you end up pulling the carb take a look at the t-slot exposure. If you have a lot of slot showing below the throttle blades the effective range of the transition circuit is limited.