Gimmee a break, my very similar combo would idle with the Idle-timing down as low as 5 degrees, no problem. 12 to 14 is all I've ever run, cuz that is where my T-slots synced up at.
Lemmee splain something;
Your AFR at idle is created by the combination of transfer slot exposure underneath the primary throttle blades, PLUS idle-mixture screws, augmented by fuel-level. If you close the throttles to reduce transfer fuel, you will have to increase the mixture screw fuel. If you do not, the engine will idle poorly or stall. So then, there is NO POINT to shutting off the transfers!
In your case, with the transfers as good as shut off, it won't idle very well if at all, on just the mixture screws, so you HAVE to throw timing at it to help out.
But now, as soon as you step on the gas pedal and open the throttles, the transfers come back on line, and now the ENTIRE low-speed circuit is fat cuz of the fat mixture screws.
That's just how simple it is.
Here is your game plan, the very same game-plan I used on mine;
1) Take the carb off, drain it, and flip it over.
2) Making sure the throttle is closed, and using the speed screw, adjust the transfer slot exposure to dead square. After that is done, do not touch the speed screw.
3) Reinstall the carb and set the mixture screws to; in the middle of their operating range. The Edelbrock/Carter should be about 2.5 turns out
4) Fill the bowls with gas and start it up, then let her warm up.
5) Now, replumb the VA to the sparkport.
6) after she is warm, the idle at 16* initial will be too fast. RETARD the stinking timing until the in-gear idle-rpm is in the window of 650 to 550; and in Neutral is not more than 750.
DO NOT touch the curb-idle-speed screw!
7) twiddle the mixture screws for best stable idle.
8) shut the engine off. Go check where the the mixture screws ended up.
a) if they are open further than 3.0 turns, the engine wants more idle fuel, so open the transfers, using the speed screw, about a half a turn, reset the mixtures to 2.5, and go back to step 7
b) but, if the mixture screws are at less than 2.0, then the engine is getting too much from the transfers, so shut them up a half-turn on the speed screw, reset the mixture-screws and go back to step 7
9) AFTER you get the mixture-screws synchronized to the transfers, you can then set the IDLE-Timing anywhere you want.
10) IF your new setting creates a tip-in sag, (see note-1), timing will NOT eliminate it. Most likely, modest additional idle-timing will not affect it at all. Usually this happens because the T-slot to mixture screw synchronization is off, the transfers being too far closed. BUT, I just detailed how to set the sync, and if you did what I explained, then the sync is now right on, and DO NOT mess with it! If the sag is very minor, ok, with the speed screw, open the throttle 1/4 to 1/2 turn, twiddle the mixture screws back to best lean idle. then see what happens.
If you have to, increasing the WET fuel level, will make it easier for the fuel to climb up the mainwells.
11) to see how close your AFR is (obviously without a gauge),
here is the procedure;
>Determine exactly where your after-tune idle mixture screws are set at,
should be 2.5 turns out, +/- , 1/4 turn.
Warm up the engine,
rev it up to about 2000/2200, and set the fast-idle cam to hold it in that neighborhood.
twiddle the mixture screws lean, to obtain the highest rpm.
kick it off fast idle and shut off the engine.
determine where the mixture screws ended up.
a) if more than 2.5T, then the engine wants more fuel at the rpm of testing.
b) if less than 2.5T, then the engine wants less fuel at the rpm of testing.
c) once the load comes on, she will want additional fuel, perhaps 1/4 turn.
d) if the screws are within lemmee guess, say in the window of 2.25 to 2.75 turns out, then forget about it. But if say a full turn, then I would consider fixing it.
e) you only have a couple of options to change this, and this will affect the life of your plugs, your fuel economy, and if very fat, can shorten the life of your rings and cylinder walls.
Note-1
A tip-in sag occurs when the transfers are too far closed. In this condition, air passing by the throttles sorta evacuates the upper exposed section, and then when you gently open the throttles, it takes a split-second for additional fuel to get pulled up and over the top of the mainwell. This is felt as a slight hesitation. The further closed the transfers are, the bigger/worse the hesitation becomes.
Do not confuse this with an accelerator pump issue, and do not try to cover this sag with pumpshot! Doing so costs gas-money and engine-life; Fix the sync.
Note-2
AFAIK, all hex-shaped Chrysler VAs are adjustable.
Inside the nipple, there is an allen screw, that adjusts both the onset and rate of advance.
On the arm is a number that indicates, in distributor degrees, the amount of advance each can is calibrated to. I have seen anywhere from I think 4* to 15* which has to be doubled, to get crank degrees.
HOWEVER, the thing that sets this number is the large stops on the arm, which you are free to grind off, bit by bit, until you get whatever amount of advance that your engine wants, up to a maximum of 22/24 degrees.
It's so easy, anyone can do it.
Mine is set to bring all of it in, as quickly as is possible, no delay ...... which allows me to easily run 14* or less idle timing, even down to 5*, cuz as soon as that circuit wakes up, she brings up to 22* to the party.
Note-3
btw, if that cam of yours is on a 110 or less LSA, it has the potential to make great to fantastic hiway fuel economy.
With that cam in straight up, the power extraction is very long, compared to other cams of this size. By combining this long extraction period, with the high cylinder pressure of your 340, your engine has the potential, with optimized cruize timing, and coupled with an overdrive; to go deep into the 20mpgs, assuming that 340 is not pushing a brick..
However, the overlap period is generous enough, that with headers and good heads, it should continue to make modest power to past 5500.
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As an aside;
My Hughes 223/230/110 cam had very similar specs, and easily went deep 20s with overdrive and a manual trans. I used to mention that, on one particular trip, point to point, geared 65=1600, she made 32mpgs; and most everyone here seems to think that is impossible, pretty much calling me a liar......... so if you hear that, from somebody else, just know that it was done on a day-long trip, and we were not always doing just 65. By 85mph, that rpm had risen to 2100. The timing was optimized at cruise rpm, using a dash-mounted, adjustable, electronic timing module, with a range of 15 degrees.
My combo has never burned anything higher than 87, except for the time she went to the track, and ran four Eighth mile blasts, with a lil hi-test 91 dribbled in.
Furthermore;
that 223 cam, in my 11.3Scr/367 used the same plugs for 5 years, until one day the lobes fell off the cam. I replaced that cam with a 230/237/110 cam, and left those plugs in it.
Those plugs were new in 1999.
They have seen three cams, several intakes, several carbs, and three ignition systems ......... and they were still in there over 100,000 miles later. I finally installed new ones in fall of 2022, as a courtesy to the guy that wanted to buy the car. So those plugs were 23years old when I pulled them. Jus saying.