What I think you have is secondary butterflies in your intake manifold.
If you remove the carbs you can see them in there. They are normally opened
by an arm on the carb and it may be that the arm has disappeared or been
installed wrong. What I have done with good results is to remove the butterflies
and the shafts and then plug the holes. (I've used sawed-off 5/16" hex head bolts
installed with sticky permatex and they worked well.) You'll need 4 bolts and
they should be long enough that the threads don't reach all the way up under the
bolt head. Saw them off to about 3/8 or ½" of shank and glue them in the holes
with the stickiest permatex you can find. Check the length before you glue them
in - you don't want them extending into the intake passage.
If you can get your hands on a low-pressure pressure gauge you should check your
fuel pressure. Should not be more than 3 psi. Too high a fuel pressure will make
the carbs leak.
You may have to check the float valve setting or replace the float valve in the
offending carb.
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George Downs, Bartlesville, Heart of the USA!