When you have the engine pressurized put the transmission vent line in a bottle of water.  If you see bubbles it means the leak is some where that is connected to the internal space where the transmission lives or is remotely connected to the vent line ( use your imagination  to construct of all of the possible surfaces internally that share the volume that is connected to the vent line).  This space is connected to the case halves parting line,  the right crank seal,  the three 3mm holes that support the iron main bearing sleeve during the casting of the cases but are fill with epoxy from the factory, and the seals on the power valve that are suppose to prevent exhaust from getting into the transmissions living space.

It would probably take a drop of more than 3 psi per minute to lean it enough to make the idle hang at 2000 rpm or so...........or make it act real lean at an idle.