I would start with that ,to do that you have to go to pro Porsche shop or anybody who has a gas analyzer to set your fuel mix....this is the basics, before you start trouble shooting...
If that has not been done on your 911 you might have to remove AFM and remove the factory plug.Under there is a adjustment screw to correct the fuel mix...
look here
Also to set the correct fuel mix you have to measure before catalyst-if you have one on your 911...and oxygen sensor disconnected..which you can actually leave disconnected -i do not know how is in Finland with smog tests every 2 years?