ORIGINAL: aussiesteve
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.
aussiesteve,
In a life many years ago, I use to drive a gas tanker truck. It was not uncommon to get quite a buildup of what looked like a very fine, clear silica sand in the bottom of the tank compartments. Yes, this material would have been downloaded to the underground storage tanks at a gas station and in those days, there was no final filter in the hose before the nozzle that goes into the vehicle .... so it would have been passed on to the vehicle tanks in some quantity. Maybe something like that could be happening these days and if so, maybe that fine material is what you are seeing in the screens.
I never could understand where that material came from but it had to come from the pipeline facility I loaded out of.