Hi all. I'm having a nightmare with my 312, I just can't get it to idle smoothly.
Here's the history..
Fully rebuilt 312, everything new, done by myself ( a professional engine builder) . Isky E4 cam, pertronix ignition in a new distributor, new Edelbrock 500cfm carb.
Right from the first run , it would not idle properly. Eveything has been swapped. Complete ignition system, new coil, new leads, new plugs, all wiring checked for voltage. 2 different carburettors. Valve lash is 20'' hot. Ignition timing 10 to 12 degrees.
The engine will only idle rich to the point it stings your eyes at the tailpipes. The motor shakes as if it's misfiring. Sounds like it is but all ignition system has checked out good on a Sun machine. I renewed everything again just to be sure.
Compression test shows 180 psi on every cylinder . Cylinder leak down test showed 5% leakage on all cylinders.
Above idle speed motor runs smooth and pulls strong.
Anything I have overlooked? Spent all day on it today and got nowhere.

Sounds like your doing everything right.  I know Rick has a similar 'setup' as you and was running a 465 Holley I think he tried and tried with standard jets [about 58's?]  It wasn't until he jumped right up to low 60's might have even been 64's then it ran really nice, hasn't looked back. See if you can scab a 600Holley and try , don't be shy on going up jet sizes.



"Came too close to dying to stop living now!"

The leak down test sounds high for a new engine.  The leak down test itself should have helped to clarify where the leakage was occurring.  Some of your symptoms points to worn valve guides.
 
Here are some other ideas.  Were the valves hand lapped in place after doing the valve job?  Were the ring end gaps checked?  Cast or moly rings?  Was the cylinder wall finish appropriate for the rings being used?  Were the heads cc’ed?  There’s the possibility of one cylinder head having larger combustion chambers than the other.  Where is the camshaft installed in relation to TDC?  I’ll assume the camshaft was degreed in and not simply put in at 12 links between the dots.  I’ve found camshafts off by as much as 18° when simply installing the timing chain sets without actually degreeing them in.  Easily fixed during the build process but can be a tuning nightmare if off significantly and in the vehicle.
 
How’s the manifold vacuum reading on a gauge?  If not steady and high, then start looking at the valves, guides, springs, and lash settings first.  The richness from the carb is potentially from too much initial idle setting thus exposing too much idle transfer slot.  This could be symbolic of not enough ignition timing, a carburetor that’s too small for the application, inadequately sealing intake valves, or improper valve lash settings.  This is assuming the float level in the carb is set correctly and/or the fuel pressure is not excessive.  Edelbrock carbs are sensitive to fuel pressure variances.

Lorena, Texas (South of Waco)

Thanks Ted, a lot to think about there. I could definitely hear valve leakage into the intake on the leak down test, on every cylinder. I didn't do the heads myself but I trust the guys that did them. They were new valves and new guides.
I will check the cam timing also, I can't remember what I set it to, it was 2 years ago. I have lost the cam card, what should I be looking for timing wise?
I have 2 carbs here, a 390 Holley which it ran fine on before the rebuild and a brand new 500cfm Edelbrock performer. The engine behaves the same whiscruber carb is fitted.
Thanks for your help .

Vacuum leak ?

That's what i thought , but I can't find a leak.

Wouldn't a vacuum leak cause a lean sittuation?

Durham Missouri

Did you check your spark plugs to see if they are getting sooty that would be evidence for a rich condition? I have an E4 It idles around 800  to 850 w/std transmission. The cam has a bit more overlap than a stock cam which I attribute the rough idle I have. If you do have sooty plugs It could be due the additional overlap the vacuum is fluctuating enough at idle that the power valve is opening too early or the pistons to the metering rods are doing the same on the Edelbrock. That being the case a change in that area will help, The Isky E4 I put in straight up and it was checked on a degree wheel to be accurate with the 12 pin factory setting could have been a luck of the draw. At that setting it runs good and has a good pull in high gear. My Isky cam data calls for .015" hot for the valves. 

Thanks everyone. I've just put a timing chart on the front pulley, tdc it, and checked at the inlet valve pushrod with a dial guage and it's come out at 106 degrees. I don't know what it should be as my cam card is misprinted and is unreadable. Is it ok at 106 or should I start tearing the front down?
I think the problem could be as you say, carburettor. I have fluctuation on the vacuum guage. I expected a slight rumpiness at idle with this cam but it's not a consistent lope. It's more of an erratic misfire which occasionally smooths for a couple of beats then misses again . It's enough to shake the car. Listening down the Venturi the air doesn't seem to flow smoothly , I can hear gurgling.
Off idle the engine is a smooth as silk and pulls strong.
Plugs show soot around the base of the thread but the electrode itself is a nice mid to light brown.

Mike,
My Isky data shows Intake open 22 deg BTDC and close 58 deg ABDC. That would put the intake centerline at 108 deg ATDC. Depending on your accuracy you're real close and because your reading is close you must be fairly accurate. If your reading is perfect you are 2 deg advanced which is fine. This is assuming your intention was to determine the intake center. With the compression
readings your getting my guess is that you are a bit advanced. I don't think your cam position is your problem.