Ignition timing may certainly be an issue. We don't have specs on what
degree the timing should be set to. Our emissions application manual
indicate you need to refer to the underhood "emissions label". Timing will
absolutely effect HC (hydrocarbon) on your Subaru Legacy. Some other factors
that come into play are the CO and CO2 readings. CO2 readings should measure
around 15% indicating complete combustion. CO should not be too low. Low CO
maybe causing a lean misfire resulting in high HC as well.
First thing first... Check the ignition timing.
