RAise the float level.
Decrease size of idle air bleed.
Only two ideas I have.

 

................................

 

Part number 37 is the venture cluster. Arent the 2 little tubes closest to the outer edge ,on the underside, the Idle channel restrictors?
Cant you open them up? Or ........?

 

OK so opening them up, say .005" at the time will richen up the idle?

 

That is what I am thinking. I also agree with IMSPORT . If the upper air bleeds are downsized, it will cause more fuel to be pulled into the Idle passage .
It would stand to reason that a 2 barrel from a larger motor, say a 400, would have larger tubes and channel restrictions. Got any other carbs laying around?

 

Recognized HTML document



Paragraph 3
.......................

 

Turned out the timing was retarded and was heating up the headers red hot and allowed the A/F meter to read lean(??). The motor had one of those spun aluminum harmonic balancer covers that apparently was indexed wrong. The car had a bad flat spot just off idle, advanced the timing and the flat spot went away, lean idle went away.