1966 Plymouth Valiant /6 & Holley 1945 carb can only get to idle at 1250rpm. How to get a lower idle without dying?

So you went from one carb to another and still had issues. Make sure you go through all your vacuum lines as it could be a leak somewhere causing a problem with both carbs.
No, I rebuilt the 6145 carb and am still having the issue. vac lines are good. I ordered a rebuild kit for a NOS Holley 1920 I found at a swap meet so I’ll let you guys know how it goes.