That fitting is not long enough in the threaded section to enable the flare seat to contact the taper inside the carb fitting.
There are longer thread length fittings available in most auto parts stores....
I have used teflon tape on the threaded sections only in the past....but you would be better off with a longer thread section fitting.
You could also look for a fitting that has a flat seal area and a copper washer to seal against the flat section of the carb inlet fitting.
