I am converting my 4780-2 (800 DP) to four-corner idle.
So far, I have successfully modified the secondary metering block for idle screws/passages, surprisingly without messing anything up!
Also drilled (deepened) the as-cast holes in the main body holes to connect the new idle passages. Now it's time for reassembly!

I bought a new Quick Fuel 12-700 baseplate with passages for 4-corner idle already, rather than try and drill tricky angled intersecting holes...
But it has .028 "dribble holes" beneath each secondary transfer slot which are non-adjustable sources of idle (and coasting) fuel that I'd rather not have.

Can I plug those with lead shot at the bottom of the transfer slot bore, to make the idle circuit identical to the primary side?
Or should I leave them be?
(Engine is a 451 with 272@.050 cam, not much vacuum and will need plenty of fuel too).

I've got a PM in to Dom (Thumper) already.
Thoughts?
thanks.

I would, and do, drill that passage for #6x32 threads and buy some blank #6 x32 brass set screws and use them to plug that passage up
That way if you need to add more idle or transfer fuel through that passage you can remove it (they are allen head) and drill as needed and reuse it

Thanks. I really didn't want to drill on my shiny new baseplate though
I heard back from Thumper and he agreed that plugging the dribble hole was the way to go.

So that is what I ended up doing... said to myself that that dribble hole is needed on a vac. sec but not a four corner idle double pumper and I want all the idle mix to be adjustable.
If I need the idle even richer I can always make the air bleeds smaller
It's all back together and on the car. Primary T-slots square, will set the idle with the secondary screw. I notice the secondary T-slots are higher up the bore than the primary, so right now they are not exposed.

Will try it tomorrow maybe, if something's messed up it is a nice afternoon so I quit while I was ahead!

I also drilled the center of the main body 1/4" straight through from the air cleaner stud hole, and drilled/tapped the upper part of the stud mount boss for 5/16.
My plan is to use a brass male (5/16)-to-female (1/4) thread adapter, screw the stud into the adapter, and drill bypass air holes in the adapter if needed, rather than the throttle plates.
Thought about a hollow stud but the walls would be too thin if I need as big an air hole (4 x 3/32 = one 3/16) as I did before.
And I didn't want to drill/tap at angles inside the air horn or near the radiused entrance to the venturis - could affect the flow, and I could imagine a set screw backing out and falling down the intake.