[Texasdoc]

Stalling: Your stalling on decel isn't because of that missing insert. Maybe if it was the idle air bleed, but not likely since it is the main bleed. Do you have the vent tube "whistle" inside the fuel bowl? Get the whistles if you don't have them. You can also try to lower the fuel level a bit more - 1/16 to 1/8 inch.

I found this on the racingfuelsystems.myfunforums Holley carb section: Rapid deceleration will cause fuel to spill from the secondary main nozzles. This is why most secondary bowls have a lower sight plug than the primary and why many OE Holleys have a secondary PV, so the MJ doesn't need to be so large.

Fuel spills out the main nozzle, not the vent tube under hard braking.

The other likely/potential cause of stalling under decelleration is a lean idle. Do you have access to a WBO2 meter? If not, try adjust the idle mixture screws. Start by turning one idle mixture screw out by 1/8 of a turn. If your RPMs increase, turn the other three out the same 1/8 of a turn. If the RPMs don't increase, turn that first screw back in 1/8 to its original position. You can also use manifold vacuum if you attach a gauge. Find the setting that gives you the highest RPM or vacuum with all 4 screws the set the same. Then start going slightly richer until the RPM/vacuum starts to drop off. That way you are on the slightly rich side of best idle. A tiny bit rich can make up for inexact timing and helps cool the intake. Again, make sure all 4 screws are set the same. You can check this with the motor off: start with one screw at a time, run it in until it seats lightly - counting the revolutions. Then make a mark on the mixture block so you can reference "zero" visually. Now back that screw out to its original position. Next do the other three individually. Make sure they are all equal. Now if you turn one 1/8 of a turn, you can visually see that the other three need to be turned.

Brass insert: I think you are supposed to have those brass inserts in all four holes. The inner two are the high speed air bleeds (main bleeds) and the outer two are the idle air bleeds. My Quick Fuel carb has screw in bleeds that are changeable. You may be running very lean if the carb was tuned with the brass insert but now it is missing. Is this on the primary side or the secondary side? If it is on the secondary side, you may not notice the lean running except at WOT (depending on how heavy your foot is) which is concerning - you do NOT want to go lean at WOT.

Three potential "fixes" for the missing insert:
1 - If it is a new carb, send it back and get a new one. I suggest calling Brent and telling him what you want.
2 - Take it into a "shop" that works on carbs. Let them tune/fix it.
3 - Do it yourself. Get some micro drill bits and a digital caliper. Use the drill bits as gauges and find out how large the main air bleed is on the one side. Then order from Holley or make the press-in bleed and insert on the side that is missing.

Either way, you want to make sure you aren't going lean.