I just worked on a friend's 1972 Mobylette 50V.
It has a Gurtner carb on it.
It appears to be a "slide" type carb.,similar to a Mikuni.
This usually means that the adjustments are jets, located by removing the float bowl.
The only adjustment you may see on the exterior is likely the idle screw.
The"bog" you mentioned is most likely the result of one of the jets being either dirty, or being gummed up with varnish from old gas.(very common)
The jets can be removed and cleaned then re-installed.
While the jets are out is the time to blow compressed air through all the passages.
Good luck Dave