Poor mileage and black smoke at high rpms

I have a TVS Radeon 110 cc commuter motorcycle. After 4th free service it's making black smoke at high rpms (when I pin the throttle) and giving mileage of 41 KMPL against the claimed mileage of 69.3 KMPL. I know mileage depends on a lot of factors, but my commute generally consists of cruising at 45-50 KM/H so, I don't mileage should be that far off. Regarding the black smoke, I think it could richer mixture, but my carburetter's fuel screw is set at 0.75 turns out (was 1 turn out earlier) from fully seated. What could be the problem? I can't properly tune the carburettor due to lack of tachometer on the motorcycle. what should be the setting of the fuel screw. via /r/motorcycles https://www.reddit.com/r/motorcycles/comments/ei117d/poor_mileage_and_black_smoke_at_high_rpms/?utm_source=ifttt