  We present a numerical method to estimate the lensing parameters and the
Hubble constant H_0 from quadruply imaged gravitational lens systems. The lens
galaxy is modeled using both separable deflection potentials and constant
mass-to-light ratio profiles, while possible external perturbations have been
taken into account introducing an external shear. The model parameters are
recovered inverting the lens and the time delay ratio equations and imposing a
set of physically motivated selection criteria. We investigate correlations
among the model parameters and the Hubble constant. Finally, we apply the codes
to the real lensed quasars PG 1115+080 and RX J0911+0551, and combine the
results from these two systems to get H_0 = 56 +/- 23 km/(s Mpc). In addition,
we are able to fit to the single systems a general elliptical potential with a
non fixed angular part, and then we model the two lens systems with the same
potential and a shared H_0: in this last case we estimate H_0=49_(-11)^(+6)
km/(s Mpc).
