It is the ign timing, too much. It is doing two things: increases the oxides of nitrogen & increases hydrocarbons.
For the test, he should reduce init timing to about 0-4* BTDC & disconnect MVA if he is using it.
Does make some sense I suppose. I had assumed retarding the ignition only help when a catalyst is involved, but a very well heated pipe/exhaust can work very well as an afterburner too, even without a catalyst.
