Jan, thanks for the update.
Did you make the header yourself from 58cc parts? What muffler? It looks like 3204, but the 3204 muffler only has 25mm inlet, and I think the 80cc will need 28mm inlet diameter.
I am vey much expecting your results.
I did some checking on other 80cc engines, and very good results were obtained with MTW TD110 mufflers. No data on MVVS 80 yet! I have one 80cc engine left, but do not want to use it for testing because Lennart may need it.
Please keep us posted.
PS,
Here are the dimensions for the flange in jpg. If I print at 49%, I get the dimension just right off my printer.