I found retarding timing increased HC but decrease NOx (my HC nearly doubled going from 5* to 1*-2* BTDC but NOx went down by 30%). Advancing timing does the opposite. Adding ~10% alcohol can help with HC, adds more O2 to the mix and that can help burn more of the unburnt HC. I usually use plain old ethanol (aka shellac thinner at the hardware store).

r.c.brown@ieee.org
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