What causes Carbon buildup?

[larry bud]

Having to clean out the throttle body every now and then, I was wondering what actually causes this carbon build up? There's no ignition occuring inside the throttle body (or before it), just mostly ambient temp air passing through it.

So what's the deal?

 

[Tuna]

The intake valve does not seal completely every time, the intake/exhaust overlap allows some exhaust to flow back into the intake, a little pre-ignition or after burn in the cylinder, post shutdown exhaust flowing back into the intake, backfire.

There are lots of reasons that carbon gets into the intake.

 

[Ken]

Air is dirty as well, and you also have the crankcase breather routed to the throttle body.