That fitting with the big nut is also a holder for a fuel strainer on most carbs, and there's a thin nylon gasket on the inboard end of it. If that gasket happens to be missing or damaged, no amount of tightening (including stripping the threads in the carb body) will stop the leak. Try a cordless 3/8" drive impact wrench with a 1" socket to GENTLY persuade it to come loose so you can inspect/replace the gaskets.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!