Carbon Buildup?

[goggleman]

So I took the bike in to the shop to check on some weirdness in the motor after the rejet. Turns out one of the valves was jamming because of carbon buildup.  

Now I am just curious what could cause enough build up to clog a valve so it can't close on a bike with less than 10k miles on it. Machanic told me lugging the engine a lot might do it?

[The Buddha]

I am guessing the vlaves were just staying open from having become tight. Jetting = richer mix = more crabon and lower temp, but I still dont see how it can buid up and keep a valve open... Rpm's do clear these things out though.
Cool.
Srinath.