Good catch on the gasket hole! Lots of people miss that one.
With that ruled out, I would suspect an accumulation of varnish in the idle circuits. What is the history of the gas tank?
The volatiles in ethanol gas is an excellent solvent for the many years of accumulated deposits on the walls of the tank which would normally have stayed in place. Now they may have migrated into the carburetor where it is now accumulated, plugging the tiny holes.
Carl
