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