Had my pipes off the other day and took a peek at the rear,It was sooty but no build up on the valve stem or in the port.All smooth.
"Self I say, leave well enough alone,it's running fine".

I have seen the same thing with my bike too which is a common thing to see for most R*'s motors accross board. The plugs in bike look approx the same all the way arround being a nice very med to light tan and its not running too rich either getting good fuel milage showing no signs at all of being too rich.

I think what you guys are seeing is that even when your bike is jetted properly not running too rich the rear cyl runs considerably hotter then the front cyl that can cause the additive pkg in the fuel to react differenly with the significantly higher temp the rear cyl see's vs the front cyl. That situaton can cause the fuel's additive pkg to gum up/carbon up the rear cyl considerably more then the front cyl.

Think of it as for example when you heat std motor up to 300 deg or more & it all the sudden turns blk and start to gel up/gets gummy/thick & sticky and i feels thats what is happening to the additive pkg in the fuel due to the rear cyls hotter/excessive operating temp.

That's why i run Seafoam and or Chevron's Techtron fuel inj/carb/carbon cleaner in my R* 1x every 4-5 tankfuells to keep the carbon/gummed up situation from getting out of hand (esp on valvestem/s) whay can lead to the dreaded SVS.

Just my $.02 on that one.

scott