Typically this is caused by DPO having warped the air horn by overtightening it instead of replacing an old dried up gasket .

If your new gasket doesn't fix the leak you can add a super thin bead of fuel proof Permatex sealant to both sides of the gasket before assembly , let it dry 24 hrs before use and be aware you'll need to soak the carby in alcohol to ge it apart next time .

I've saved several junker carbies this way .

-Nate
There is no problem so difficult it cannot be overcome by generous application of brute force & ignorance