Hello,
Just bought 1986 LG4 bone stock with the quadraJet electronic carb. The car has 4 speed AT

The car starts easily and drives well except for an issue when coming to a stop (the problem doesn't happen when the choke plate is closed, only when the car is in operational temperature and the choke plate opens)
I set the idle to 800RPM when in P/N.
When I shift into D/R the RPM drops to 600 and the idle is stable, then I can drive cleanly until slowing down to stop, at this point, the idle drops and the motor dies.
I can immediately restart and put in D/R and the idle is stable at 600RPM.
I can also during the stop just before the engine dies to hold just a bit of throttle, then after the car fully stopped I can release the throttle and the engine idles well while in D.

Seems that the problem is the transition just before the car fully stops.

Any idea where should I look?

If you give it a little gas (like we all did when carbs were the norm) when it stumbled, you were increasing fuel and dropping vacuum (since the carb blades are opening). The first thing I would do is check for vacuum leaks. Next I would check the float level. I am no expert on quadrajets, but there is an excellent sticky on how to tune them found at the top of this board.
I also know there is a throttle position sensor for CC carbs. Check to make sure it all hooked up and wires are not hacked (if you are running the ECM).

Try a half turn clockwise on the idle air bleed adjustment.

Where do I find the idle air bleed adjustment screw?
I do not see any adjustment screws on the Quadrajet E4ME aside from Idle and choke fast idle.

Dan

Just in front of the choke blade is a little "tower" about an inch tall. It has 2 adjustments located in it. The rearward one, nearer the choke blade, is the IAB valve. It's down in there a ways and looks like a large brass screw.

This is all of course, under the assumption (yeah I know) that the factory-installed cover over it, is gone. If that's still there it's not that hard to get it out so you can access the adjustment.

The bogging at idle problem came down to float valve adjustment.

The document shows a measured float height of 12/32" (3/8") for the Rochester M4MED on a Canadian 1986 Pontiac.
I measured and remeasured it and it was 12/32, yet the car will bog down when slowing down at idle.
​​​​​​I dropped the float to 14/32" and now the car runs perfectly.

I had a similar issue on the quadrajet on my 69... Fuel spillover to primary bores causes the mixture to get super rich and the car dies... It can be high fuel level, bad gasket, or warped air horn from cranking down on the two front mounting bolts...

Giving it a little throttle gives it more air and the mixture leans out... Easier to do with a manual... Giving it throttle and brake in an auto car is trickier... And risky...

Yes, I was driving it with my both feet for 2 weeks, certainly trickier... And risky..