Doubtful it's an issue with the Air Gap design, many people run them without this issue. Which is not to say your specific manifold doesn't have an issue.
Years ago I ran into an engine that had the two outer plugs on one side and the two inner plugs on the other side all carbon fouled. All four plugs were fed by the same half of a QJ. The carb was in need of a rebuild.
Look at commonality issues is my suggestion. You realize the three cylinders you are having issues with follow each other in the firing order? Did you look at that?
I had two bad plug wires on my similar build running E Street Heads 75cc Angled Plug
They where carbon fouled on the terminal end where it clicks on the Spark Plugs
Also thats why i asked if he opened up the carb and checked everything and he said he did
In his initial post he mentions Slight Misses At Idle ???
As in ??? Fuel or Ignition related
Also are you running any type of carb spacer or 4-hole gasket with that intake manifold ???
