Well,
if it is a 600 vacuum Holly(can't remember numbers,plus I'm too lazy to look it up, but I'll take Larry's word on it.
I could never get a Holly 600 vacuum carb to run good.
It just seemed too big, which is was.
I could get a 600 DP to work better but that still,was too big a carb.
I am no Holly expert, but I have played around w/them long enough to say that a 600 is too big for a 250 mild build Chevy 6.
The main jets do not effect idle, there is an idle circuit(sp).
I think you should look for a smaller carb .
Maybe Larry has had a good carb combo to run for a single four barrel setup?
I just got spoiled after switching to side draft DCOE Webbers, I would never, I mean never ever go back to a four barrel.
I always had good luck w/the old Carter AFB's, Edlebrock makes them now.
I bought a tuners kit for the AFB carbs, you can change the springs, jets, metering rods etc..
Good luck!!
