OK I'm in trouble; I have never worked on a 4-corner idle carb.
What I want to know is how do you know the Idle is rich?
I ran the 292/292/108 in my Hi-compression 367 for one summer with OOTB Edlebrock Alloy heads, an AirGap, and an ancient 750DP.
I ran it at 14* Idle advance with a manual trans; and the cylinder pressure was over 185psi. After setting the Transfer slot sync, as previously described, I had to introduce some Idle Bypass Air. I tried cracking the secondaries on my DP, but without fuel in that bypass air, it idled kindof crappy. IMO, the bypass air has to come in close to the Idle discharge fuel ports. So to that end, I did this by drilling ONE tiny hole in each Primary throttle blade. I started at 1/16, and went up by 32nds. I ended up at 1/8, but that was too big, the transfers dried up on me before I knew what was happening. So I filled the holes with solder, moved over, and started again. I spent a lot of time at 5/64, but IIRC I ended up at around 6or possibly at 7/64ths.
With a 4-corner carb, you can just crack the secondaries, and adjust the secondary idle-fuel screws, as may be necessary.
My guess is that you might be able to set them all the same, but I really have NO IDEA.
The principal thing to remember is this;
On the Primary side, there are THREE fuel-ports to deliver Idle fuel, namely;
1) the transfer slots,
2) the Idle-discharge ports, and
3) right beside those are the trimmers which are adjustable by the "mixture screws".
All of those circuits are modified in the Fuel-wells by Air coming down the bleeds.
Idle-air is supplied by the cracked Primary throttle blades PLUS the PCV system, Plus whatever bypass air you introduce on the Secondary side.
I idled that cam at ~750/800 with a manual trans, and IIRC the Manifold Vacuum at 14* timing was a tad under 10 inches.
To get the low-rpm timing back, I installed a two-step curve from a smog 318. That spring set has two different springs, one of them has a long loop. This spring does nothing until the rpm gets up to about, in my case, 2800. So the regular spring allows the timing to advance fairly fast. I had to be careful to not allow any mechanical advance below about 900rpm. From there the timing jumped linearly to 28* at 2800rpm, then slowed down and the all-in was 34* @3400rpm., which was by the springs combined. As you might guess, this took me all summer to achieve.
To get the cruise timing, I modified the V-can to 22* by filing the stops partly off. I think you can do this to any 70s V-can. My Eddy-headed 367 liked over 56* of cruise timing.
There, that will get you started.
Your 340 may, of course, have slightly different requirements.
And, OOps more coming, the wife is calling for supper
And let's not forget;
1) in at 104* that cam has an Ica of 70*, which leaves you just 110* of compression time
2) The overlap period is 76 degrees.
3) the power-stroke is just 102*
4) with the piston at TDC on the overlap phase, 42* are allocated to the intake stroke and 34* go to the exhaust. With BOTH valves are slightly opened at this point. The headers are supposed to be encouraging the AF charge to get moving from the plenum into the Combustion chamber, before the piston gets started on that job. But with the piston now at TDC, the header will continue to pull on the intake until the exhaust valve closes some 34 degrees later.
What this means to your engine is this;
If the headers are working really well, they will pull some of the fresh F/A charge straight across the piston and into the pipes. If your headers are sucking air at the heads, then fresh air will also be in there. If that fuel ignites and burns, the expanding gasses can easily feed back into the combustion chamber thru the open exhaust valves.
When the exhaust valve finally closes, the piston is moving down and the atmosphere begins working earnestly to fill the cylinder. When the piston hits bottom and turns around, it will be another70 degrees before the intake valve closes, soooooo the piston is well on it's way up when that happens. At low rpm, the piston will push some of the just-inducted A/F charge back into the plenum, thus reducing your Idle-vacuum.
The thing of it is, with a 3.315 stroke and the intake closing at 70* ABDC, your EFFECTIVE stroke is reduced to 2.43 inches. That means that the piston is .885 up from the bottom when this happens or ~27% of the way.
I tell you all this, just to say, that there is a whole lotta stuff going on inside your intake manifold. So I ask again; how do you know the Idle is rich?
BTW:
What is your local elevation?
