wimwerf,

The air filter was probally a bad example but an engine will wear with use and
my explanation is still valid.

You cannot expect an engine to be EXACTLY the same after use.
It does not take much to upset the AFR at IDLE because of the relatively small
volumes of air and fuel required.

Andother example is the fuel injectors do clog up with use which is why you must
clean them regularly aswell.

Ther are MANY factors !!