Dialing in a Carter AVS
as in screwed in far enough to almost make the spring go solid)
What you are experiencing is lack of idle air bypass.
The engine wants more air, so you crank in the speed screw, which makes the transfers flow too much so you close up the mixture screws ....... So you crank up the timing, to back out of the transfers, and bring the mixture screws back on line. What you are doing is chasing your tail. Everybody does it.
What you need to do is set the transfer slot exposure to be in the ballpark, like to a lil taller than wide.......... and leave it alone. Later on, much later, you may have to fine-tune it just a smidge.
Then set the mixture screws to mid-adjustment.
Then set the timing to whatever gets your idle-speed to where you want it to be; without regard to what it actually is. With an automatic, it doesn't matter what the timing is,until she hits the stall speed. And you can adjust that with rate-of advance ,and the Vacuum-Advance at Part-Throttle.
Then fix the idle AFR with a combination of bypass-air and a slight mixture adjustment, that could be had with
a) a tad more or less transfer, or
b) a tad more or less mixture screw adjustment
c) a tad more or less bypass air.
d) occasionally, the airbleeds for fine-tuning.
Big cams want bypass-air, the bigger the cam, the more bypass-air it will want.
Your 274 cam will want bypass air equivalent to drilling holes in the primary throttle blades of about 5/64 one in each. With that Carter I know of no better way to get bypass air than to drill.
When you get it right, the engine will idle right down to 650 in gear. With a lil less ignition timing, it might go down to 600. If you can get the M-rods to stay down, and with a lil more finessing,and with a heavy flywheel, I see 550 in gear, with a bit of a tendency to not like it, but she'll do it...... at ~5* advance, and with the right AFR. Whether or not she'll take throttle,lol, is another matter.
Just leave that transfer slot exposure alone. That is your main low-speed fuel supply... and your main idle-speed fuel supply. The mixture screws are just for enrichening the idle but if you set them really rich, then, since they never shut off, that means your entire low-speed system will be rich. Try to keep them at 2 to 2.5 turns out.
If you drill the bypass holes too big, the idlespeed will go up, which will require the timing to be retarded more. and to smooth it out will require more fueling, so it's and endless search for the perfect balance of bypass air to enrichment, and jacking with the timing.
I can tell you that a pair of 1/8th inch holes will be too much, and a pair of 1/16 will not be enough,lol, and a pair of 3/32 might be right on. But I suggested a pair of 5/64, one in each primary butterfly, cuz you can play with those for a long time.
If you drill too big,don't panic. You can fill them with solder, move over, and start anew; No big deal. Unprofessional for me, but not so much for you,lol. Just chamfer the holes,both sides and make sure the solder sticks. Then after it cools, dress the slugs smooth.
