What's a good ballpark figure for the idle air screws? How many turns out from the bottom? The car I just bought runs really rich. It's a 600 cfm model. I'm probably going to take it to a Mustang shop in Chandler but that's a few weeks out.
Just a thought, but most of the time when someone has trouble with a new Holley 600 vacuum secondary, it's usually not the carb at all. Check the ignition, both timing and overall condition and compression and I'll bet your carb gets better. Stock points ignition is shaky on a good day with a two barrel carb. Also if you added a four barrel intake to a high-mileage motor, it may not have decent compression, which means the carb doesn't function as well as it should. Outside of the choke sticking shut, engines are fairly forgiving as far as a bit too much carb are concerned.
Engine has a fresh rebuild. Hooker headers and Pertonix points eliminator ignition. The idle air screws were one turn out, moved to 1 1/2 turns. Still rich. I have an appointment with a business that has a gas analyzer to dial it in.
Had the car put on a gas analyzer. It was showing 13 to fuel to air ratio. The problem was the P.O. had the vacuum advace hooked up wrong. Now it runs great and I can hear the secondaries opening!
Glad to hear you found the problem. You did the right thing by making sure the rest of your combo was right before diving into the carb. People are far too quick to just blame the carb (especially Holleys for some reason) and buy another one only to find the problem still exists. Bet you picked up a bunch of power as well!
general rule of thumb is 1.5 to 3 turns out depending on how the thing idles.
Set your timing, then your idle speed (650ish with a stockish cam)
Then turn the screws in, CW, about a half a turn per side, go back and forth side to side.
If the idle mixture screw setting is the problem, the idle will clean up, and should speed up and run more smoothly.
likely there is something else wrong with the carburetor, like a previous back fire blew out the power valve diaphram, so you're getting fuel through that circuit when you're not suppose to be.
If it's still in print, pick up the holley carburetors and manifolds book from HP Books, GREAT resource.