Easy this one !!!

Dont forget, the cosworth ECU is a "speed density" air flow type.
I.E. it does NOT directly measure the airflow of the engine.
It relies TOTALLY on the mapping.

So over time as your air filter blocks up and the pistons reduce there seal as the
rings wear, less air will be drawn in and the ecu CANNOT measure these
very small changes that occur over time.
(There are other factors but you get the picture)
These are only small enough to affect light loads. (idle/low to mid range)


As a result, less air upsets the AFR at low loads and causes higher CO
I.E. excess fuel compared to air.


This is why cars have a lambda sensor now as these auto compensate the fuel
ratio over time for all variables that change with age and use.