OK, one or more of your carburetors isn't closing all the way FOR SURE. Never, ever, ever start up a six pack car without the outboard linkage(s) connected or you can blow the motor (front/back carbs can go wide open with no way to close them). Here's what I would do:
1. Turn the center carb idle screw all the way out (lowest possible idle). I assume you've already done this.
2. See if you can force (gently) the center carb closed any further by hand. If yes, then your return spring may be weak and/or hanging up somehow.
3. Inspect carbs for anything that may have moved & now interferes with any of the carburetors returning to fully closed (vacuum hoses, plug wires, etc.)
4. WITH ENGINE OFF, disconnect the outboard linkage & see if that lets the center carb close even further. If yes, then re-adjust linkage, re-connect & THEN start engine to test.
5. While the linkage is disconnected (see 4), manually move each carburetor by hand gently to see if any one of them is hanging up. It is very common to have a carb/intake gasket hang up the throttle blades on six packs. (fyi-always best to move carbs open/closed while tightening them down to prevent this....nudging carbs/gaskets so each carb moves freely before fully tightening bolts)
If none of that works.... report back & we'll think of other possibilities
1. done
2. can't
3. will double check
4. did/done
5. never thought of that will have to check
my gut feeling is that issue probably lies with base gaskets. vacuum hoses all hooked up correctly as far as i can tell. got new Holley gaskets for all carbs. unfortunately had to remove all carbs eventhough i tried everything i could to not have to do so. this is the only thing that's really changed as far as materials go. everything was torqued to spec (200 in/lbs) but still just not sure everything is right there. really don't want to have to remove everything again.........
