<VV> Plugged hole mystery? - Carter YH

[Grant Young]

I am rebuilding a poor-performing YH carb for a customer and found the large hole in the venturi that leads from the idle port in the throttle body plugged with a rivet. It is the large (approx. 3/16") hole inside the body next to the idle air bleed hole. Is this a mod that I missed the discussion on? Other than an incorrect inner upper pump spring (from a Nash) and aftermarket metering rod (which have never worked for me) this is all I can find out of whack. Thanks for any insight on the mysterious plug.
Grant