AAhhhh, The 6-pack blues........ In the 27 years that I've had my Challenger, I had a 4BBL, then a 6 pack, then a 4 BBL, Now Edelbrock 1000 CFM Fuel injection. Notice how I went back to a 4BBL after the 6-pack? It was fast, but I got tired of "working" the throttle just right to keep it from stalling when I let the clutch out and when I came up to a stop sign. My carbs were worn out. If yours are new, you should be able to get it tuned up to start "working" your 6-pack. There are lots of working 6-packs and 6-BBls out there.
The white plug that you show is a cap to lock that idle mixture screw on that side. Pop it off, adjust the screws then pop it back on. Both screws should be adjusted the same way from side to side. Look on that Holley Site again and read about adjusting you IDLE MIXTURE. I bet we read your first post wrong when we thought that's what you were talking about, when you watched that guy adjust yours.
When I say CAP OFF your front and rear carbs to see if the idle changes, I mean COVER THE HOLES ON TOP. Take something that won't melt and wont break and get sucked in, like rubber from an inner tube. Wrap it over the hole and rubber band it tight. That way you'll know that they aren't allowing air in, when you're troubleshooting an idle problem with the center carb.
When you built your engine did you use a "STOCK SPEC" cam? If not, you better read up on changing your power valve to match your vacuum. If you have low vacuum at idle, your power valve could be wide open just idling. A bigger cam causes low vacuum at idle.
Rob
