Prefect feedback thank you all ! I'll try Hawkeye's drill next time.
The only part I find odd is, that there is absolutely no info about mixture setting in the AFM! As if that adjustment facility didn't exits as of writing. I'll double check but I'm pretty sure I didn't overlook it.
The AFM I'm am referring to is not some copy of "appr. a Hu269",
it is is a copy for one of the very machines I am flying!
The general lack of knowledge about that PIC operated mixture setting might explain why those two S-300Cs are so extremely different to start, and might explain the appallingly different experienced power output.