Good day,
what would be an optimal ignition timing advance for a 2.0 Douvrin running on LPG? The engine has done 140000km, 50000 on LPG. The factory value is 10 degrees timing advance. I set it to 12 degrees, any negatives in the long term?

There is absolutely no reason to do so.

Keep ignition timing the way Citroën prescripted and only adjust your spark plugs to a more narrow setting of the electrodes.
The gap with LPG must be set to 0.6mm instead of 0.9mm for gasoline. This is to protect your coils.

Your ignition system will take care of the needed advance settings during driving of your CX.

Thank you for your answer. It’s usually recommended advancing the timing a bit for LPG but it doesn’t like advance at high revs, over 3000rpm. Douvrins are popular in your country so I wanted to hear some first hand experience regarding long term consequences, if any.

A little advance is ok. but if you encounter knock at high revs it is too much. 12 deg is ok i think.

That’s what I gather, thank you. Just to be on the safe side.