Air leak diagnosis - where to begin

[TundraOG]

As a follow up to my last thread which kind of got streched out I'll explain everything here:

After a carb cleanup, new plugs and oil/filter, the bike starts fine with choke and goes to about 3K RPM. When warmed up it stays at a solid 1400 with no choke. Throttle response is poor and the RPMs return slowly after cranking the throttle. The idle screw was unscrewed all the way, and as soon as I screw it back in the idle jumps up to a steady 4K.

With the help of a friend we looked for air leaks. Took apart the diaphragm cups who are intact and show no signs of leaks and cleaned that area, took off the engine-to-carb manifolds and checked for leaks/cracks, they're a bit dry but still in good shape. The airbox seals tightly...so WHERE in the world can this air leak come from?!

Help please