While you have the carb apart, check the fuel screen with a good magnifying glass.

I have seen a few screens on "well run" engines recently that looked clean to the naked eye but under a magnifier they have a clear crystalline buildup on them. That buildup moves easily enough with a "reverse flush" of Carb cleaner through the carb orifices. I have no idea what the buildup is - it isn't the normal dirt that we see and I suspect it is just some sort of precipitate from the fluids going through it over time, I have no idea when it starts to form or how long it takes to form - I have some theories but won't espouse them here yet as they are totaly unproven.