crud forming under throttle plate

[Extent]

Pulled the carbs to get at the front valvecover today and noticed that there is some nasty black carbon looking, and some grey build up underneath the throttle plate and around the bypass circuit.  The top portions of the carb are still squaky clean, any idea what could be causing this?

[Lucky]

unless your experiencing some backfire or putt-putt, i'd say don't worry about it.  most of the carbs i clean have something of that sort in the same spot...

that's why they developed carb cleaner...

[inanecathode]

IF you were to worry about it (which i wouldnt at this point) you'd worry about the surface between the throttle plate and the throat of the carb.
Like i said, shouldnt have to worry about it, but obviously the carbs are out, you should clean the shit out of them while you have them in your hands instead of putting it off for the next time they're out.