Can't get it to idle.

After disassembling my carb, blowing out the air bleeds, and re-assembling, I finally got it running sort of properly. I can get it to idle down to about 1200-1300, and it sounds and smells nice and crisp. However, even after getting up to temp, it will suddenly drop off and not recover. It should actually idle about 900, and I've been sneaking the timing back closer to a proper initial, but the only way I can keep it from randomly dying is to keep the idle up past 1400. The carb is an older BG 750 DP, with no choke or air horn, and downleg boosters. I set the idle mixture screws at 1 1/2 turns out, as a baseline. It starts great. Half a shot of gas, hit the button, and it's running. Just can't get the idle sorted. ??

Was it OK prior to the carb re-assemble ?

Sounds like the timing is retarding causing the lower idle speed and dying
I have had the same deal, problems, on a motor with a big cam and a not real loose converter
Changing the advance springs and shorting up the mechanical advance slot fix it by being able to set the idle timing in gear at 14 to 18 BTDC and have right at 34 BTDC full advance over 1800 RPM

Usually the reason that it will idle turned up on the idle rpm is that you getting extra idle fuel from the transfer slots that it wants. You probably need to back the mixture screws out more.

All the cars I've work on that had the idle speed screw up far enough for the throttle plates to expose the transfer slots ran rich and stunk like unburnt gas out the tail pipes.
I can remember seeing a post about intentionally exposing the transfer slots on here several years back, really bad suggestion
The transfer slots are their for adding additional fuel at light part throttle changes from under 1200 RP up to around 2100 RPM
The idle circuit is tunable by using the idle air bleeds as well as the idle mixture screws Don't make it idle on the transfer slots

Okay. I had some self-induced carb issues last summer. I did solve them, but I also had the carb apart for new gaskets. I kind of overthought my first fire-up issue, solved it (crossed wires. Doh!), and then it was all over the place on the next fire up. Thought for a bit, and pulled the carb apart, blew out the bleeds on a hunch. I set the throttles at about .040 of transfer slot visible, and set the idle screws at 1 1/2 turns out. Right now, the timing's at 32 at 1500. Can't get a shot any lower, but in the past it has idled nicely at 10 initial, with 36 all in. Cab, am I reading it correct that I should close the throttle blades up until the transfer slots are NOT exposed? I also suspect I could bring the idle screws out a bit more, so I will try that tomorrow night when the neighbor's at work.

the carb is design to idle on the idle circuits, not on the transfer slots
Unscrew the idle speed screw so there is no throttle plate opening and then adjust so it has around 1 to 3 turns in against the idle stop on the throttle shaft and see how that does, most of the Holley after market non stock OEM carbs I've used and tuned like between 1/2 to 1 1/2 turns out from gently bottomed out on the idle mixture screws
I also make sure if it is not a double pumper to adjust the rear throttle plates so they are open a little tiny bit from being full close to help clean up the idle mixture some Maybe try opening them from .005 to .015 visible

How much cam? Might need to go the other way to get the idle where you want it.. Crank the initial up and take some centrifugal out to get your total.

Sometimes if the cam is big enough I have had to locked the distributor out, set the total and roll with it.

The cam is a Comp 308 solid roller. 262 @ 50. It's the same cam as I've been running before. I'm pretty sure what I'm dealing with right now is a bit of carryover from last years little carb tuning adventure. I do thank everyone for their input, as it will get me back on track.

That cam will like more timing at idle. Try 20° or more. Then adjust the Carb.

What Cuda said. Crank that initial up!

Agreed.
Fundamentally, I like to have the basic ignition curve sorted out before trying to dial in the carb.

for what it's worth i use a whole lot less cam than that 308 and idle about 25 degrees before TDC. my initial is 18 with one light primary spring that bumps the timing up and idle speed in neutral is 850rpm. this works very well for me. you're doing over 40 degrees of overlap at .050"; i do about 18. trying to make that work with 10 degrees initial would be a big challenge. like said, fix the ignition first and the carb will be easier to manage.

If my memory serves me they want a .020 feeler gauge in the throttle plates so the transfer plate has the correct amount of vacuum to pull fuel in at idle. If you need more air you can drill small holes in the throttle plate. Some of the BG carbs have an air valve you access down the center hole. smaller idle air bleeds may help. Good advice on advancing the timing as well.

https://www.motortrend.com/how-to/ctrp-0707-carburetor-tuning/

Many carb problems have been fixed by correcting the timing.

"90% of carburetor problems are in the ignition" - Smokey Yunick

This has been true in my experience once or twice. If you don't run enough timing, you'll have to turn the idle screw beyond the idle circuit of the carb -causing problems. By advancing the timing which increases RPM, or making sure you're running the proper initial timing for your engine, you should be able to idle within the idle circuit the way the carb was designed to run. This is the first thing I'd confirm before moving forward.

knowing what distributor is being used would help.

If the timing situation seemed to be close to what I thought I’d end up with(20-25 initial/32-38 total), then “my” next step is to just grab another carb off the shelf to see if a different carb solves the problem, or at the least changes the idle characteristics.

I realize a lot of people don’t have that option........ but I haven’t had only one carb on hand for many many years.

I find that swapping to “known good” parts is a pretty effective way to troubleshoot.

also a 750 carb has smallish throttle blades, with a cam that large (after timing is figured out) it might need holes drilled in the blades to allow enough air at idle and to keep the transition slot exposure small enough.

I like that one.
The distributor is an MSD unit. MSD 6AL ignition.

you can buy bushings and springs for that distributor. not a difficult modification. or, a start/retard; they work.

Years ago, had a slant six at work that gave me lots of grief. Turned out to be a clogged air breather on the valve cover that was forcing fumes into the intake manifold. Simple fix was a vented breather instead of a solid oil fill cap. Wasn't ignition that time, but points and timing is normally the first place I used to look. Malfunctioning fuel vapor canisters run a close second when I would inherit a problem child that others couldn't fix.