I’ll check that when I get home, but I don’t see how that’s possible. Just to be sure I’m right, it’s paper gasket, isolator block thingy, rubber oring and carb? Brand new paper gasket and oring were used and the nuts are nylock nuts. But maybe I had a gasket fail or that hose between the 2 carbs is cracked.
Just going by the "last thing you changed" philosophy, I'd recheck the float to make sure that the "older plastic float" you installed didn't get shocked by having to go back into service again, or that a piece of muck didn't interfere with the float needle or pivot ... ?