So mine could have been a 1979?
As far as the flat spot, couldn't you richen or lean the pilot jet or possibly change the needle position? I'd go richer first, like larger pilot jet or drop the needle one position.
Any more information on the history of the DT? Like models and years?
Out in the farmland growing up a kid had a red 175 just like the one above. My buddy had a XL185 Honda and we used to tear up the farm. Good times. Too bad my buddy died a few years ago in a car accident.