  In this paper we prove a Nekhoroshev type theorem for perturbations of
Hamiltonians describing a particle subject to the force due to a central
potential. Precisely, we prove that under an explicit condition on the
potential, the Hamiltonian of the central motion is quasi-convex. Thus, when it
is perturbed, two actions (the modulus of the total angular momentum and the
action of the reduced radial system) are approximately conserved for times
which are exponentially long with the inverse of the perturbation parameter.
