318 Idle issues

I have seen this several times and Agree with Rusty's assessment, that she is sucking air. Number one clue is that you can cover the Primaries, with no change.
For this post, I'm gonna assume the shake is NOT a wrong/faulty balance.
The following is what I would do;
1) The first thing I would do is a Compression test. Not because I much care about the actual numbers, but rather, I want to know the difference between the highest and the lowest. This is just to establish a baseline and prove all Eight cylinders are working correctly. Once I get a satisfactory baseline;
2) I would Seal the CC and put a vacuum/fuel-pressure gauge on the dipstick, to make sure there is No vacuum in it, only modest pressure. If pressure, which should be normal, I do not exceed 3>4 psi as to do so only invites trouble. If vacuum, the intake is sucking air from the bottom, and I gotta take it off, find out why, and fix it. Then when I get a satisfactory conclusion
3) With the engine idling, the PCV correctly plumbed and properly working, I would first check to see that all vacuum taps are sealed, that the Vacuum advance is on the spark-port and is NOT working at all, and then, one atta time, I would pinch off all the other vacuum devices to be sure none are sucking wind. When satisfied,
4) I would remove the carb, drain it, flip it upside down, make sure the Primaries are on the curb and the that the choke is disengaged. Then I would adjust the Secondary throttle blades to be fully closed but not sticking. Next I would open the Primaries and set them such that the Transfer-slot exposure on the underside is between square to slightly taller than wide. Next I would grab the Base gasket and make sure it actually properly fits the bottom of the carb and the intake, leaving no paths to atmosphere that should not be there. Next, I would flip the Carb rightside up and adjust the mixture screws to 3/4 turn out, and reinstall it. From this point I would absolutely not adjust the curb-idle screw.
At this time, I gotta make a few assumptions, namely
a) that the Wet fuel level is correct and remains stable, if I have to, I'll check it out later.
b) that the gas is fresh and uncontaminated
c) that the bowl vent is open
d) that the fuel tank is vented.
e) that the cooling system runs at a stable and acceptable temperature
f) that the air motion of the fan is not upsetting the bowl-vent,
g) that the PV is not ruptured
h) that all the airbleeds are clear
j) if you are using an adapter under the carb, you gotta check it, and it's gasket also for compatibility.
k) that the plugs are all clean and the wires are all properly and satisfactorily routed, and the cap is neithe cracked nor burned up nor filled with moisture.
l) that the cam-timing is at least close to correct, and that the valve lash, if any, is correct and not too tight, and if Hydros, then that the lifters have properly bled down.
and so, I come to
5) the tune up.
At this point it should idle perfectly down to 650 in N/P with as little as 5*advance.
At this point, I will not adjust the Idle speed with the speed screw cuz that would defeat the purpose of having taken the carb off to set the T-Slot synchronization! Instead, if the idle speed needs adjusting, I will use timing. The more you give it, the more idle- power the engine will make, and the higher the idle-speed will go.
As for Idle rpm, I only want enough to idle at as low an rpm as will not bang the trans when I stick it in gear. If I can do this with 750 in N/P and 650 in gear, that's what I'm looking for. I don't care what the Idle-timing ends up being. and so we come to the shake
6) to cure an Idle-AFR shake, you only have to adjust the Idle-AFR, lol. At this stage, I leave the vacuum gauge in the toolbox, cuz it can't tell me chit on a 268 cammed 318-stroker, that my eyes, ears, and nose, don't already know.
>If the exhaust is burn-your-nose stinky, simultaneously with the idle-shake and the mixture screws set to in the range of 1/2 to 1.0 turn out, then I would bet money that the engine wants some Idle-Air bypass. Proving it is easy; I just pull the hose off the PCV and stick my thumb over the hole, adjusting the airflow, until the engine smooths right out and the stink in the tailpipe goes away. real simple. More complicated is to actually make a permanent solution to this problem. The Way I do it, is to calculate the size of the hole that I'm getting off the PCV, and drilling the Primary valves to an appropriate size.
If I drill too big, the Idle speed will go up, and if too high, then the only way to bring it back down is to retard the timing. If the timing goes too low, the engine will begin to be sluggish in running up to the stall-rpm. This is exacerbated with low cylinder pressure. Ok so, I try to keep the Timing in the window of 12>18 degrees with an automatic, and 10>14 with a hi-pressure stick-car. O so now, she's running pretty good, assuming all my assumptions were good ones. Well not quite
7) she has to pass one more test. the tip-in test.
with the trans in gear, and the idle speed around 650, the engine must not hesitate, when you very slowly tip the throttle in. So then your first adjustment is to make sure the Accelerator pump begins delivering fuel, the instant the throttle moves; adjust as necessary. Next, if any hesitation remains, and the mixture screws are properly adjusted, there are only three ways to cover that tip-in hesitation, namely, with additional transfer fuel, or with increased Wet fuel level, or with the IABs. So the first check is the fuel level. the Second is to crank in the speed screw giving more fuel from the transfers....... however this has the side effect of also bringing more air to the party. Sometimes this makes the tip-in hesitation worse. Which means I'm gonna have to close up my bypass air, dammit. Moral of the story is easy on the bypass air..... lol
Ok so, whadImiss?
>Oh yeah, if you are running a drop-base air cleaner housing, you gotta make sure your bowl vent does not get too close to the lid. They say, AND I cannot confirm, that 3/4 inch is the minimum. and
after you get the Idle all tikkity-boo, then you still gotta work our your Power-timing, and Part-Throttle timing, and your Cruise- timing.

That's what I would do, if the balancer turns out to be fine.........