Disconnecting the fuel line at the carburetor will not tell you if there is a wad of crud against the inlet side of the float needle seat or if the float stop is bent so the needle barely opens and doesn't allow enough fuel into the carb for sustained high speed travel.
Put the line back on and pull the plug out of the bottom of the carb, then try your flow test again. Or, just take the carb apart and find out what the problem is. You already know you have good flow to the carb so the problem is inside.
Pretty comprehensive description here.
https://www.youtube.com/watch?v=-wBALe0MUgs