There is a 3/8" pipe tap available, but there might be another approach possible. Get a short 1/4" NPT brass pipe nipple (3/4"-1" long) and thread it into the carb housing with some red Loctite or Marine Tex sealer, then add a 1/4" pipe coupling to thread the inverted flare adapter into. The tapered thread on the pipe nipple should allow you to run it deeper into the carb body than the shoulder on the adapter will allow. It will look a little goofy, but it will probably solve the leak without having to replace the carb.
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!
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