Greetings,
I have a '79 corvette with the stock sbc 350 (L-48, not 82) that I bought for next to nothing as a project.  Upon purchase, I checked everything I could without actually driving it (No brakes was an advertised problem).  All fluids were old but clean, no signs of leaks or head warpage.  Listening to the engine I suspected a head gasket leak, and later confirmed with a compression check.

I rebuilt the top half of the engine from the head gaskets up, cleaning everything as I went.  Kept stock cam and lifters, installed new proform roller rockers.  Used fel-pro gasket kit.  Used original Edelbrock performer dual plane manifold.  Replaced stock HEI with MSD HEI.  Replaced spark plug cables with MSD superconductors.  Replaced spark plugs with bosch platinums.  Replaced edelbrock Q-jet clone (unkown cfm) with Holley 4160 650 cfm.  After rebuild compression check was good.

The first couple of times I tried to start it, it backfired through the carb.  Disconnected fuel line, ran off the starter and realized timing was nowhere near correct.  Set timing to 9 btdc as specified by Hanes manual.  Checked power valve and reconnected fuel.  Now engine starts right away and continues running as long as I supply throttle but will not idle.  I have adjusted the idle mix screws from 1/4 off seat to all the way out with no noticeable change.  I can get it to idle at 3000 rpm by adjusting the idle screw until the butterflies are noticably open, but it will drop out (no sputtering) if I adjust it down from there.  Today I had a thought, and tried two other carbs.  First the original edelbrock Q-jet clone and got the same problems.  Good start, good part/full throttle, no idle.  Then I swapped in an edelbrock 1406 from my daily driver (1985 Jag XJ6 with sbc 350) and got identical results.

Now that I've proven it to not be the carb, I'm reaching for ideas.  Plans for this weekend include switching to diamond-fire plugs and checking of all voltages going to/from the HEI.  Any other suggestions would be greatly appreciated.  

Sounds like you have a vacuum leak check all vacuum hoses as well as intake for a vacuum leak.  

Hoses look good.  All are new.  The only one I left connected at this point is the timing advance off the metered port.  All of the other ports are plugged, including the two off the manifold itself (Headlights and tranny).  Do you have any good ideas for checking for vacuum leaks?  The only method I know is the spray carb-cleaner method, which I'm a little leary of trying if I have to force the primaries open to 3000 rpm just to get the engine to stay running.  

Alrighty,
I installed new plugs today, checked the input voltage to the HEI at a solid 12 volts.  Still had the same problem.  Turned the idle screw out far enough to make the engine run at 3000 rpm (lowest I can get it without dropping out).  I took a can of carb cleaner and sprayed around the carb/manifold gasket, both manifold-to-head gaskets, and the manifold vacuum tap for the headlights and tranny.  There was no response from any.  I then tried a control and sprayed past the primaries and got no response.  Best I can figure, at the airflow and fuel load at 3000 rpm, the carb cleaner isn't volatile enough to make a noticable difference in RPM.

So, does anyone know any other ways to locate a vacuum leak short of replacing all the gaskets and hoping it goes away?  

sound like still possible timing problem. loosen distributor and turn it a little one or other till it runs at idle. it sound like it is just a little off. book isn't always 100%. and have had that work a few times before.  

check your cam timing....and if you left the brake booster vac line on you could have a bad booster  

had same problem once.couldn't find a vacum leak. the leak was on the engine/valley side of the gasket used a different brand of gasket with a sealer. problem solved

it sounds like you might have the same problem  

You hit it right on the head, spud. (No pun intended).  I figured it out by running at 3000 rpm, and pulling both breather filters at the same time, then cupping my hand over their seals.  Felt suction on both sides simultaneously.  When I pulled everything back apart, and re-cleaned the sealing surfaces, I felt a little raised area where I apparently damaged the head/manifold interface during reinstallation.  My guess is this were keeping it from sealing.  Guess I have an excuse for new heads now!

Thanks guys.  

The best way to pin point a vacuum leak is, start the engine- with engine idling, spray some Chemtool or Brake Cleaner around all mating surfaces of the intake manifold and all vacuum hoses. If there is a leak, you will notice in a change of RPM when you spray it. Another cause of popping out the intake and/or carb, could be timing, and or the engine needing a stall torque convertor other than stock, or possibly a bigger intake manifold.