Probable causes:

(1) threads in carburetor housing are now worn TOO DEEP, due to overzealous previous owner overtightening of the fitting OR the use of one of the forms of Teflon.
(2) the carburetor casting is cracked at the fuel inlet, due to a previous owner using some form of sealant/lubricant (Teflon is a wonderful lubricant).

Possible solutions (if you are dealing with a 1949 truck as indicated in your signature block):

(1) Remove the aftermarket Rochester B and install an original Carter type W-1 carburetor.
(2) Remove the aftermarket Rochester B and install an aftermarket Carter YF.
(3) Replace the air horn on the Rochester B with a new old stock or good used one.
(4) Machine a new brass fitting with slightly oversize threads. This fix will work for (1) but not (2) of the causes. This fix normally reserved for use with EXPENSIVE carburetors.

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop