Here's the scenario:

'71 307, open plenum intake, Carter AFB Comp Series carb, HEI, headers, stock cam.

Pulls @ idle, i.e.,900 rpm, 15-16 in/hg, steady. With alittle adjusting of timing and mixture screws, they do have an effect, 17 in/hg. Problem is there are three dead cylinders, #2,4,&5. Temp gun shows 120-140*F, all others are 400-450*F.

Pull plugs, they are not black, brown in color, I changed them anyway. Timing light shows spark for every cylinder at its plug. Cylinder compression check shows 125-133# for all eight. Removed valve covers to witness all rockers going up & down.

If I bring rpm up to 3000-3500, then vacuum comes up to 20 in/hg and dead cylinders begin to fire and engine smooths out. Let it idle, 3 dead cylinders. Carb has been disassembled and cleaned, sprayed manifold & carb and found no vacuum leaks. Place hand over air horn, engine dies.

I believe carb may be too big, or engine needs a can of Engine Restore to produce more vacuum to have carb function properly.

Opinions?

I think I would check for a voltage leak from the plug wires. At night, or in a dark garage arcing to the block etc. might show up.

Everett I believe that the carb or the restore will NOT help. Take a close look at your cap. The cylinders you mention just happen to coincide with the location of the HEI coil mounting screws. I have seen screws that are too long create slight cracks in the cap and cause an engine miss at low rpm when there is a greater KV requirement. that at 3000 rpm under no load. Just my 2 cents Good luck and best wishes.
Milan