Help! K series problem driving me mad!!

[poorpeet]

Any advice welcome on this one.
My wifes Rover 416 Keeps failing the MOT on exhaust emisions. (High co & h/c)
On the advice of my local Rover dealer I have changed the cat & Lambda this made no diff! The fuel pressure, temp sender and all the electronics are spot on as is the compression. It doesn't rattle or smoke & the oil level does not need topping up between changes.
The only thing that has made a significant difference has been adding a flushing agent to the oil then putting in fresh oil. It is now ok at idle but way too high at 3000 rpm. Oh its also had new plugs & rotor arm.
Any ideas?

[Andrew Noakes]

High CO and hydrocarbons suggests to me that fuel isn't burning correctly. So either there's too much fuel or 'not enough' spark.

I'd run it for a bit then pull the plugs one by one and see if any of them was wet with fuel.