[1] WD-40 is flammable in aerosol form, so if there was air leakage where it is sprayed, one would expect a change in idle.
[2] The increase in rpm from holding a hand over the carb is most likely due to the low idle vacuum. Not necessarily an indication of a faulty carb.
[3] The key to this strange problem lies in the low vacuum. I believe the carb & the ign timing, as the cause, can be ruled out. Enough tests have been done to eliminate these suspects.
[4] So back to low vacuum. Is it low because it has a performance cam? Or is there a mechanical problem with the engine: worn cam lobes, leaking valves, broken rings etc, causing low vacuum. A compression test might help, if it could isolate particular cylinder.
