Convert to replaceable idle air bleeds?

Before drilling, are you measuring what is in there now? :poke:

A reference for the locations on a 4150/60
Holley Carb Passages Identification Pictures

Map out the entire carb.
Holley Carb Sheet
not sure if that speadsheet is still downloadable. If not, find some posts from "GNTKllr" and cut and paste his maps for a start.

Tapping the air bleeds is a pain with the choke tower in place. One reason I suggested experimenting with wires first.
Machining main body for screw in air bleeds

making set screws into restrictors
drilling restrictors

Alternatively, the air bleeds can be removed and an aluminum, brass or slteel restrictor can be pressed in. I've done that on carbs with the choke tower

A demonstration of why PV enrichment should not effect idle mixture. This is the way it should be when all the restrictions etc are in good relation with each other.
Effect of an open or closed PV on idle

An explanation of why PV opening can cause a change in idle mixture is in Shrinker's posts here
Power Valve Question

Your older carb probbaly has the IFRs in the lower position.,
If so, keep them there. Although YR I think I thiunk has done better with them high. But he also is further along in tinking with carbs.... I would keep it low in your case so you are making the least changes
Relocating the Holley Idle Jet

Lots of good stuff in here. Racingfuelsystems - Stickies
You can join, but you do not have to. We all HATE the Tapatalk takeover of the hosting service. If you don't want to join, just close the box[X] and continue browsing like normal.