Chuck,
It sounds like you may have an internal leak. Two things come to mind that can cause this. The first is that the gasket for the power valve can sometimes not center and fuel will leak past the power valve. The second and one which caught me awhile back is that the metering block to carb body gasket can leak if there is any warpage in the parts. This is especially a problem with the reuseable gaskets. I assume that the float levels have been checked and are right.
Good luck and if you are still having trouble you can bring your carb over and we can set it up and test it on the mustang.
Forest
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