Interesting, my Sud on single carb had it's MOT last week and while it passes it has higher hydrocarbons than I'd like but what is normal on these?
last year mine was CO 3.8% HC 475 ppm
this year 3.3% and 560 ppm
which presumably suggests the mixture is now a little weak, or there is some other issue like Dr Alfs list, plugs leads are due for replacement so will be doing that soon.
The other thing is I suspect is a lot of these old boxers burn a bit of oil down the valve guides even if they are otherwise in good order. And also what effect does the crankcase ventilation have on HC? I've never been quite sure how it's supposed to work on a sud. On my other old car with a Ford V8 is so simple to understand, the air goes in one valve cover, filtered by the air filter, and it comes out of the other cover via a PCV valve and on into the intake. Presumably the valve stops it burning stuff at idle which must help the HC. The sud justs seems to have this pair of pipes with no valve.