im not sure its an induction leak, as the effect only seams to be with 1 cylinder, the other fed by the same carby has no issues at all, unless the induction leak is at a point in the manifold past the split. apart from the rpm range, everything is fine...
what makes me think its an ignition issue is the fact that when on 1 mag, the cylinder EGT drops rapidly from cruise temps to way less than half. though the strange thing is the same happening at a set rpm with both makgs on, but the drop, while noticeable within a minute, is nowhere near as rapid as with 1 mag off.