  We study some generalized instanton algebras which are required to describe
`instantonic complex rank 2 bundles'. The spaces on which the bundles are
defined are not prescribed from the beginning but rather are obtained from some
natural requirements on the instantons. They turn out to be quantum 4-spheres
$S^4_q$, with $q\in\IC$, and the instantons are described by self-adjoint
idempotents e. We shall also clarify some issues related to the vanishing of
the first Chern-Connes class $ch_1(e)$ and on the use of the second
Chern-Connes class $ch_2(e)$ as a volume form.
