Cleaning exhaust headers.

[Gingersdaddy_imp]

OK, The fresh air is introduced to extend the burn of the un-burnt fuel mixture that did not completely burn in the combustion process. You had a lot of this on the 80's era cars with the belt driven A.I.R. pump. It help burn the fuel off before it gets to the catalytic converter. It is totally an emission issue. Carbon build up is not seen as much with unleaded fuels of today, you still have some build up, but not as much. The valve stem oil seals are on the oil side of the combustion chamber and are not affected by carbon build up. They are affected by age, not changing your oil regularly, and seals just wear out over time. Unless, however, if your are referring to the valve seats.
Did some of you notice when you swapped out the muffler when you let off the throttle you had some cool back firing going on? When you changed out the muffler you altered the flow of the system. The pair valve was doing its job, and the un-burnt fuel was popping off in the muffler instead of being burned in the catalytic converter.
Sorry guys, I've been to school on this and use to teach it.